You'll want to track down a dealer that carries the latest
Amiga Developer CD. Then head over to Motorola's
semiconductor site and grab the 680x0 documentation (check the Design Support link). Finally, head over to
Aminet and search for an assembler. I don't have my dev CD handy to check, but there might be an assembler on there as well.
GoldED is a popular text editor / IDE.
Trev