Developing for C or Pascal using any of the THINK environments would be easy on an SE with 4 megs of RAM. No need to worry about doing assembly unless it's something that interests you, and if you do end up programming something that requires the extra speed, the inline assembler in THINK works fine. Lots of examples and tutorials out there on the web too.

MPW is very powerful, but I haven't been able to find any earlier versions that don't require 7.5.x, which as someone has mentioned is a hefty OS for an SE.