Hi newbcake ,
I've made a few robots, but the only one I brought as a kit is the Pololu Zumo (Chassis only), a bit small but that's the point of it. My first one was very simple using micro-switches as sensors, then one on a round perspex chassis, a Pololu Tracked RP5, etc. control has included L293 drivers, servos and ultra-sonic's for distance and obstacle finding..
Build it yourself and save a packet, especially if like me you don't have it in the first place... The Tamiya dual motor gearbox is a good start. I don't find 2 wheel robots a problem at all, in fact tracked bots have problems turning on carpets. Visit http://www.banggood.com/Wholesale-Smart-Robot-And-Accessories-c-2745.html for a very cheap £6 kit.
It all good fun, but a few other more useful things are my seed propagator controller and fish tank controller..
If you care to take a look here: www.melsaunders.co.uk you can see some of my stuff, and my other interest!! I will help you (or anyone else) anyway I can......
Regards
Mel.