The Walker Bulldog would be a good chassis, but you might want to check the possibility of using the sprocket, idler and tracks from a Pz III. Use those and add a 4th return roller and the suspension should be OK. The Cromwell Cruiser is a boxy thing, so some sheet styrene and the turret base and ring should get you the turret, but still, a big job and lots of scratch building. Doing the Mk IV? I've done a number of these in 1;35 but never had to do one from scratch.

Bill