Any. Make sure you get 10.2 for it if you're going to be making commercial apps (general rule of thumb - always target the lowest common denominator).
I would say "no" to this advice. With 10.4 on the horizon, you might as well plan to target 10.3, then retain that compatibility when 10.4 ships.
Plus, the latest wxMac works the best on 10.3 or higher (I think).
3) The generated apps are forward and backward compatible
with the existing Mac models?
I believe your apps will always be
forward compatible. Backwards, like Ryan Norton said, is a different story... especially if you want to target OS 9. (My advice and that front: it's Probably Not Worth It)
I have been told that in OSX, gcc is included along with the Mac similarity to the M$ devstudio.
Yes. The developer tools may or may not be included in the package. If they aren't you can sign up for a free online developer account at
http://developer.apple.com
Witch type of IDEs/compilers/tools are required for "standard" Mac applications developing? E.g. for Windows I would suggest:
You can do your work with gcc/make or XCode. If you need to target OS 9, or want a higher quality (less crashy, etc etc) IDE than XCode, you should try Metrowerks Codewarrior (
http://www.metrowerks.com). It's a bit pricey, but mostly worth it.
A development environment is really all you need. A basic icon editor is included in the development package, and if you need a separate editor there's BBEdit (
http://www.barebones.com) or I think there's a Mac version of UltraEdit now.