Great start! You might try some of the car/truck gear for your wheels and differentials. I saw some RC 18 wheelers not to long ago that had military style wheels on them. I too will be following as you get your feet wet!
Hi Contact Forgebear and see if he can get the front axels for the half-track(Im pretty sure they are the same size you want), also the gearboxes if you remove the sprokets could fit a set of drive wheels(would only be 2 wheel drive),just a thought he gets spare parts for the Sherman.