Right on, we can make some more assumptions using these specs. You can get a battery based on calculation using motor draw and runtime. DO that after you decide what motors will work for this project.
I will do that 
Calcs look good although I'm not sure where you got a couple of your constants. 8cm radius is a rather small wheel?
Yes I know - the wheels on the electric wheel chair radius were only 8cm, that's why I originally used that in the calculations.
Whenever you're able. Honestly the easiest way to go about this is to make a simulaid out of wood and add weights to it. There's no substitution for real world experimentation.
Okay, I will try to build a wooden model in a 1:20 scale or something like that.
PM me when you're not busy with school and we can work out the details. I'm interested in building a balance bot too - now is as good a time as any.
Okay, I will send you a PM next week.
Regards
Lauszus