Geez, buying the parts seperately....the list goes on and on.......
To buy the parts for two gearboxs you can buy the whole kit for a few bucks more.
another route is to watch some ebay auctions for some stock clods and harvest the gearboxes then. Buy bearings for it afterwards if necessary.
You can put TMax wheels on a clod if you want.
http://www.thundertechracing.com/adapters.htm
although these guys have killer wheel and tire combos that fit the clod without adapters.
http://www.imexrc.com/
If you go dual servo and are serious about crawling, then 4WS would be good. But, otherwise, a good 2WS setup is 100X better then the stock 4WS. with that said, I still like running my daughters stock clod with the stock 4WS, it gives a certain thrill when the rear end swings around like a forklift!