First of all, good for you! learning C is a great experience and can offer you alot!
1) What is the best C (or C++) package for Amiga programming, and where can I get it?
IMHO, Storm C
You get it from Haage & Partner.
It is a really good product, especialy for begginers.
There is also GCC.
2) How compatible are the various packages? Could I, for instance, compile source written in GCC with SAS/C and vice-versa?
If you are talking about writing your own software, it's up to you.
If you write using nothing but plain(old) ANSI C, it should be very compatable.
C is ment to be compatable. Just don't use specific compiler conventions.
3) All the includes and everything that came with SAS/C 6.3 seem to be for OS1.3 and 2.x. I run OS3.5 so do I need some sort of SDK from somewhere or do I need a more modern version of SAS/C?
The 1.3 and 2.x stuff will work. However you would be best to get the Amiga Developer CD (2.1).
But for learning, using the 2.0 includes is fine.
