The difference are in the valves sizes IIRC...however, every tuner will say their stage 1 2 or 3 will consist of certain things while other will say their same staged heads consist of something else etc...

But as far as im aware...its the difference in exhaust valves where the difference lays...stge one is standard sized valves with a slight port and polish

stage 2 is bigger inlet valves and better head flow

stage 3 is bigger inlet and exhaust valves and same porting and polishing as stage 2 or the ports are even bigger than stage 2

Every stage can consist of uprated cam however its the differences i explained above that are the main differences...but yes you can use your existing cam no probs

yes you can go for stage 2 head and standard bottom end...i know someone who was running stage 3 head and stage 2 turbo with stock bottom end and his car went very well!

There will be a chance of det when the head is fitted as there will be a difference in air and fuel going into the cylinders so its best to get everything checked over once its all fitted

bassboy