Hi
Thats great info John has given you and I just about always buy 4 or 6 function decoders (may sound like a waste but you never know when you need a couple more). I have a class 66 running 10 functions, 6 which are lighting.
It depends on the lighting on the DMU. If you only have 2 operational lights, red and white get the 2 function decoder, and function decoder but the 4 function is handy if you want lets say cab lighting etc.
You might also want to ask John about his flicker free kits for the interior of the DMU. You can light the full passenger section and the lights will not flicker when you go over points or lose power. Very nice piece of kit and adds that extra dimension to your DMU!
Hope that helps.
m