Hi John,
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:
http://www.armorama.com/modules.php?op=modload&name=Reviews&file=index&req=showcontent&id=6326
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:
http://www.britmodeller.com/forums/index.php?s=3c3ac87a67da063b91468eb5f041b645&showtopic=67430&mode=linear