The best free C compiler is VBCC
Perhaps, but probably too slow for 680x0 hardware
I tried it a few years ago and it was too slow for even a simple 'Hello world!' program
For 68030, Dice is quite usable speed wise, and perhaps SAS6.58 (which is probably much better than Dice).
For assembly coding there are AsmOne and AsmPro
Indeed, these are absolutely fantastic for learning 680x0 assembly language.