I'll try to give a response that's not technical but rather revolves around why you asked this question.
Apart from the slightly different rules in pretty much everything, from grappling to sundering to how damage reduction works to the levels at which you get some powers and some spells, 3.5 is sort of a big errata.
The main changes were fixes to spells such as harm (harms an enemy by touching him, leaving him at just 1d4 HP, with no way to avoid it), haste (too strong effect for casters, too strong defensive bonus), polymorph (no limit on the powerfulness of monsters you can become, only size matters), several "all day long" spells reduced to "choose which to cast during combat."
Most 3.0 material that has not been revised could be ported as is and not cause problems, but much of the rest is just broken under 3.5 rules. I'd suggest that, if a 3.x experience is what you are looking for, you play 3.5 or even Pathfinder RPG. Since you're buying 3.5 material you can read most of the rules in the System Reference Document (SRD).
You'll notice much has changed (for example, how the size of a creature determines its dimensions on the battle grid) and 3.0 material might be awkward to use unless you have a good knowledge of both systems. Such as the knowledge of a 3.0 player who moved to 3.5 over time.
Also, remember that even in 3.5 a lot of revisions have been made. The same spell could have been published in different books and be therefore unbalanced if the book you own is not the latest. Unbalanced seldom means unplayable though, but navigating your way among the books is hard if you're starting from scratch.
Anyway, the SRD and any 3.x edition's PHB are enough to play the game (the PHB contains wealth by level tables for PCs and NPCs, the XP table and everything about leveling up).