I think to be successful working on these engines the most important thing to possess is PATIENCE. If you're starting with a decent set of tools you should be fine but at some point you'll find that you need a special tool. This is when you need to exercise some self control and patience. At this point you should stop what your doing and seek out a tool that will make the job easier.
From a mechanical standpoint, the M104 isn't that much more difficult to work on than the M103. Access is better on the M103 but the basic layout of the block is about identical. Obviously, the head is different and the fuel injection is different.