I bought both Fallout and Fallout 2 almost a year ago because I'm a big fan of classic isometric rpgs. I, however, never got around to playing them and wonder if I should go about playing them without using a guide/character build first. I'm just wondering if there are some character stats/perks/skills that are completely useless and shouldn't be invested in.
Thanks!

Nope its just a pain depending on what setup you have. If your a melee build your golden but low strength, low melee combat ability characters will struggle through the beginning of the game.

Unfortunately, you're starting only with your fists and a spear, so if you're not specced for either Unarmed or Melee combat the Temple of Trials will be a pain. That said, if your agility score is high enough, you should still be able to get through without taking too much damage via the clever use of hit-and-run tactics.

Is there any penalty for taking a long time in Fallout 2? I'm still quite poor so I've been using the rest until healed option and it has been making the days fly.

I believe that unlike the first game, Fallout 2 has no time limit.

Instead of using the "rest until healed" option use the "first aid" and "doctor" skill as often as you can instead. Even with only 15 skill (=success chance) your average healing speed will be faster than when you rest and you'll also get extra experience. If you're still injured rest for 24 hours, you can use the healing skills again afterwards.
Don't worry about time limits, even if you rest for 10 years in the beginning you shouldn't have trouble finishing the game.

naked.
(someone had to do it)