@wormspeaker:

You might search around teh internets for some of the skill challenge brainstorming. A lot of people have come up with great ways to turn them into a more structured RP venue with XP awards, which I think is the real strength of these constructs. I agree that these are a new idea and are very rough, but I think its been a neat effort to get DMs thinking about how to structure and reward RP encounters. Try here:

http://www.critical-hits.com/features/skill-challenges/

@nexis:

I would probably recommend 4.

The advantage of 3.5 is it is a very flexible and open-ended system even if you buy only the core 3 books. The main reason is that you can very freely pick and choose between the classes easily, which gives you a huge number of options for character types. The downside is it can get very complicated, particularly once you start dealing with skill heavy classes and higher levels.

The advantage of 4 is its very easy to learn, the rules are consistent between classes (no separate rules for caster or skills frankly), and the monster templating is easy. The downside it is far less flexible, but only because it is a lot more like 2e in that the classes are well defined and you are pretty much stuck with what you pick (at least with PHB 1 + 2).

Also, you can find (almost) complete 3.5 rules anywhere... search for d20 SRD. Here is one:

http://www.d20srd.org/

@GP:

As far as I can tell, the power sources are purely flavor... they have no impact on the rules, and you can easily and effectively change the style of a class only by changing the power source and reword and slightly rework the skills. Like changing a barbarian into a swashbuckler or a ranger into an archery soldier.