I got Dungeon Siege for free (game design class in college), and I gave up after the second "town". I was playing a Nature mage, so one of the first things I looked for was a Combat mage to round the party out. The first town had no Combat mage available, and the second town was in ruins (and had no Combat mage available). At that point, I quit.

Also, the Dungeon Siege editor was horrendously buggy. It's the sort of software that, in order to truly learn how to use, you need to learn what is safe to do and what isn't. An experienced user will avoid the options that make the program crash, but a new user will not know that and will get extremely frustrated really quickly.

Also, this is one of those games where your movement relies on pathfinding (no way to control your character directly), and pathfinding doesn't just work for custom maps; you need to be careful when making your maps so that the pathfinding works correctly.

There is joinable character at very end of 1st dungeon, who can be trained as Combat Mage. DS has unrestricted class leveling, and in early game :Lv.1 Fighter/Ranger/Nature Mage can easily be changed to Combat Mage. Also game seems better be played in closed multiplayer for respawn and New Game+.

DS II included respawn for single-player too. Eventually you could become Lv.100 in all 4 class archetypes though it would take a lot of time.

On the subject of DS1/2, any good recommendations on mods to go with it. From moddb is can see:
DS1:
Revived - Adds additional things to base game
Elemental - Totally new

DS2:
Legendary - Adds additional things to base game

And there is also Lazarus which is an Ultima 5 remake in the Dungeon Siege engine at its own web page:
http://www.u5lazarus.com/

Anything missed (particularly graphical updates, does ENB work?).

For first time playing, I'd go only for base game. You can search for mod which removes Gold Cap in DS1 or you won't be able to buy gear which is priced at over 10 mils (such gear exists in original game). If you want to play multiplayer way (with respawns) you will need to adjust monster power, because it is nerfed in comparison to original game (I am not sure where I did put those fixes).

DS2 is to be played only with Broken World. Even with BW it still has some bugs unfortunately. I have fixed some of them, just rename attached file to "Fixes.ds2res" and copy to Resources directory. It fixes 1 specific high level piece of armor missing "0" in Item Level, it fixes 1 notorious undead archer hero and some similar things. Sadly I couldn't find how to fix Books and Bestiary (1 Lore Book is missing in Broken World and you can't fill Bestiary because some monsters never spawn because of shared spawn with other monsters).

I don't remember case with DS1, but DS2 modifications link your characters according to game's control sum, so if you will be installing some other modification, you will have to make new characters each time, so think carefully. There is way around it, but it is very unelegant.

All in all, original game >>> mods, unless they are bugfixes.

Ah, I have played both through several times. I like to try out mods.

The Witcher series are great games. but if you like the turn based games. I would recommend The Eschalon serries.

Diablo 2 with the widescreen patch is fun, especially as Sorceress,

Does that work with other mods? E.g. Median XL.

Probably not.

I would like to mention Jade Empire. It`s on GoG, it`s fun, it has action packed fights and you can even use the "Drunken Monkey Stile". What do you want more? Oh, and it has a story, which doesn`t fit on many other action RPGs.

I can definitely recommend you to try Torchlight that's here on GOG!