45 mph isn't really a problem,pretty easy to achieve in one of these trucks.Any where from 8 to 12 cells is where you want to be.How much do you want to spend on the system? also think about how many cells you want to run,the packaging is a little difficult in these conversion because of the space,i think with 12 cells you end with a saddle pack that's stacked.
Did you see this thread
http://www.rc-monster.com/forum/showthread.php?t=3337 these cells fit well in the MT2s,that's like a 60mph setup though,about as much power as you'll cram into one right now,unless there are some square shaped lipo cells i don't know about.