I'll try to give a response that's not so much tecnical but revolves around why you asked this question.

Apart from the slightly different rules in pretty anything, 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 fizes to spells such as harm (harms an enemy by touching him, bringing him at a stupid amount of damage from death, no way to avoid it), haste (too strong effect for casters, too strong defensive bonus), polymorph (no limit on the powerfulness of mosters 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, many otheer things are just broken under 3.5 rules.
I'd suggest you, if a 3.x experience is what you look for, to play 3.5 or even pathfinder.

Since you're buying 3.5 material you can fetch most of the rules in the srd document at www.d20srd.org
You'll notice much has changed (for example, how the size of a creature affects its dimensions on the battle grid) and 3.0 material might be awkward to use unless you have a good knowledge of both systems. This usually means being a 3.0 player who passed to 3.5 over time.

Also, rememer 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 last. This seldom means unplayable and orienteering among the books is hard if you start from scratch.

Maybe you can comment with a list of books that are for sale so we can aid you discern the 3.0 ones and the obsolete ones.

Anyways, the srd alone is enough to play.