I managed to obtain Masterclub wheels and replacement tracks, so can go ahead with the 1962 Soviet unit as originally planned. Two questions:

What kind of aftermarket machine gun to stick on?

What sort of figure to come out of the round hatch in front? Uniform and so forth.

This thread is very helpful about uniforms:
http://www.red-alliance.net/forum/index.php?topic=26562.0

Soviet infantry uniforms change little in the first decades after WW2, so theoretically you can use a WW2 figure. But you'll have to change his pouches and swap his rifle (Mosin/PPSh) for SKS, AK-47or AKM. You get the idea from the pics in the thread

From the pictures (of a BTR-50) that I've seen, the headwear of the MG-gunner seems to vary a lot. Some wear an infantry helmet, and some wear tankist helmets. So I guess it's entirely up to you what he should wear.

Tom- just wondering what version of the 50 you are planning? The standard P or PK could mount the SGMB- but I don't believe this was fired by a crew member, it would have likely been a soldier in the troop compartment. I believe on the versions armed with the heavier KPV MG it was mounted differently, over the Commander's cupola if I remember correctly. If it was an un-armed version then a soldiers could fire from the open compartment- probably with an RPK armed soldier at the front.
As for crew members uniforms it would likely have been black overalls for the driver- depending on the era these could be very similar to WW2 versions although by the 70s these had changed to different patterned versions. The Commander, depending on the unit, could also be wearing black overalls but if he was leading the troops in the vehicle, say in a Motor Rifle unit, he would likely be wearing their uniform.