If you wanted a whiffy fire-support vehicle on a Panther chassis, how about putting the 150mm L12 gun from the Brummbar into the front of a Jagdpanther? The Brummbar's weakness was always that it was too heavy for it's Pzkpfw.IV chassis, so this would give you the same firepower with the Panther's excellent mobility. Pretty easy model too....
For a Jagdpanther-based APC, you could usefully knock a couple of feet off the height of the fighting compartment. The BTR-50, which is a perfectly useable APC of similar, rear-engined configuration, is some 700mm lower than a Jagdpather.