I generally use a Scholar (GM ID Item, extra XP) and a Teacher (extra XP) early, swapping out the Teacher for an Instructor once I don't need to be as stingy with money. The Instructor provides a better bonus for your XP, but it takes a fair bit more of your gold.

If I really need a particular thing (like I want to Town Portal somewhere, or I want to fly out to the islands in Avlee), then I'll hire that particular class, use their spell, and dump them. One other hireling I'll pick up is a Horseman when I'm farming Arena victories; the horseman removes almost all wait time between hopping out and back into the Arena and lets you hit the Arena every Sunday instead of every other Sunday. I dump the horseman as soon as I'm done with the Arena.

My rules of thumb:
-If I can't go ahead and learn it right now, then I don't buy it since I might stumble across a free copy as treasure. Exception: if the spell is sufficiently useful and getting a copy on-demand is enough of a hassle, then I'll buy a copy for later (ex. Lloyd's Beacon can only be reliably bought in Evenmorn Isle, which is an annoyance to go visit).

-I look for spells that do things that might otherwise use a consumable item. Cure Poison, Cure Disease, Stone to Flesh, and Remove Curse are all good examples. I'll consider duplicates if the relevant condition will prevent the primary caster from fixing the problem; only one character needs to know Cure Poison, but a cleric can't cure their own paralysis.

-I look for spells that provide a lot of utility and/or make my life as a player easier. Wizard Eye, Fly, Torchlight, Water Walk (if I don't already have Fly), and Town Portal are all insta-buys if I can afford them and use them. Note that only one character needs to learn these spells for them to have their full effect, so I don't buy multiple copies.

-If the character can cast spells, especially if they are primarily a caster (sorcerer, cleric, or druid), then I'll pick up one or maybe two cheap direct damage spells. Acid Spray, Harm, Deadly Swarm, etc. This gives the character something that can finish off a monster on demand, or if I just need to make sure to lay out some damage.

-Once money is no longer a concern of mine, I'll fill out everything missing from my spell books. Even for classes like the Spy.

Every shop keeper buys and sells for some percentage higher/lower than the item's actual cost (this varies by area). The elven areas have a much less consumer friendly difference. To see this in action, pick a fairly valuable item. Check the sell price in Harmondale, then check the sell price in Avlee. You'll get a much better price in Harmondale. Generally the stronger areas have a higer markup/markdown for buying/selling.

If you have GM Merchant, then every shop keeper sells at cost, so this is no longer a concern.

This is pretty much correct. You can find better stuff in stores, but there is no need to buy anything in stores before you get GM Merchant (or Master, if you can't get GM). Once you can buy and sell at cost, then go nuts with any upgrade you can find because you don't lose any money on the transaction.

Once you start enchanting things and turning in Stalt ore, you'll start seeing very little turn over in your equipment.

I generally hold off until I've completed some promotion quests and done some other side quests. You can go ahead and rescue the dwarves from the mines, just don't turn them in until you've got some levels under your belt. I seem to recall that being in the upper 20's should be sufficient. Don't do all the quests, or you'll find yourself scraping for things to do while waiting for some of the next timers to run out.

Which skills to invest in depends on your party composition. For example, only a thief should bother to level Disarm Trap. I'm unlikely to go hunting on Youtube for your particular channel, so just list what character classes you have.

Don't spend any skill points on Stealing, regardless of your party composition.

I do Erathia after Harmondale because it so easy. The various fountains and Globe make the map almost a Tutorial Map. After that I head to the Dwarfs. That puts me at around Level 10 to 15. The Barrows is a good challenge at that level. If you want a higher level, Tatalia makes for a good site to level between L15 to L20.
However, you may miss part of the preliminary skirmishes between the humans and elves if you take a lot of time Leveling up. It may keep you out of Harmondale too long.
By Level 15, the mines with Missing Dwarfs should be easy (though wands for Fire etc may be needed depending on your party composition.)

I've never had trouble with $ in the game. But purchasing anything from the Armorer or Weapons smith does seem a waste in the game. But the Barrow Downs replenish quickly and are rich pickings if $ is needed.

As long as you don't turn in the dwarves you've saved, you shouldn't miss anything (head to Harmondale right after turning in the dwarves). Or are there skirmishes before that?

This is true. Most areas in the game refresh after about 2 game years, but the Barrow Downs refreshes after about 6 game months.

I've gone from "Release the Spy" or "Find the Plans" mission to the Judge is Dead ("nothing suspicious here. Please move along.") This means you miss the Trumpet quest if away from Harmondale too long. And the Trumpet quest is the only way to make Harmondale Independent --- which I found out on this site. Thank you.

"Release the Spy", "Find the Plans", and "Acquire the Trumpet" all occur only after you've turned in the dwarves to get your castle repaired. "Acquire the Trumpet" doesn't occur until after the timer on the other two quests expires.

I think you the notification about the Trumpet when you enter Tularean Forest. I don't remember if you get such a notice when entering Harmondale (I don't think so, but I simply don't remember).

So level up before turning in the dwarves (or after turning in the trumpet) and you'll be fine. You can use measured leveling as a quick way to pass time that you don't need to complete the above quests, if you're careful.

@xgambit, I want to check out your Let's Play on YouTube, but I can't find it. Are you under the name xgambit there? How about posting a link?

It has been a long time worth of games since I've gotten the Trumpet quest, despite my efforts ... I always seem to miss it these days (like with my current game). I must have stayed away from Harmondale for too long.

Bookwyrm says Pierpont triggers the Trumpet quest. So now I am not sure. I have the modification loaded so I cannot test which.

The trumpet quest is on a timer, and it won't be available at all until after the quest timer for the Spy and the Plans has run out. I don't remember how soon after those quests have expired the Trumpet becomes available (it might take an extra month or two), but I'm pretty sure you only get the notification about the trumpet when you enter the Tularean Forest area AFTER the trumpet timer has started. I think the Trumpet is only available for a month or so of game time.

I'm thinking about playing through again myself, and I can check this.

I'd be interested in knowing. I even kept going to and from the Tularean Forest to check, and I still missed it.

It will probably be a few weeks before my free time lets me progress that far, but I'll kick up a new game soon-ish.

Any party composition suggestions? ;)

Depends if you want to go Light or Dark. My current game (drawing to a close) is T/C/S/M Light, and with swift weapons for all, they're pretty much demolishing everything in their path with little loss of HP, except for the occasional Storm or Blood Titan (they're really annoying). But you could try for C/S/S/D or other caster combination for Dark. (I've run S/S/S/C (as well as K/K/K/K) in MM6 and had similar destructive capabilities.)

*shrug*. I've beaten the game a half dozen times or more, so I'm open to pretty much any group. For my last run, I used a random number generator to pick races and classes. I don't have a preference for Light or Dark, either.