I'm only basing this on the SKP Challenger I built and comparing it with the Cromwell and Comet so may well be wrong compared to the real deal...
1. I won't comment on the tracks as the SKP ones are supposed to be rubbish. HOWEVER, see here for more info:
2. It's longer so is probably a full size 17pdr.
3. Same cupola and hatch as the Cromwell.
4. Leaf spring, slightly wider than the Comet one.
Lastly, there was a discussion that may help you over on BritModeller: