The motors will be spinning at the same rpm since they're on the same spur gear.
If your using 12 cells on each esc now, I'd use 2 4s packs of the same age and specs. If you use 1 4s pack on each side you won't be able to sit some of the batteries under the chassis like you do now obviously, but I wouldn't want to do that with lipo anyway.
Your chassis looks like it will hold more than 6 nimh cells length ways, so if you have about 165-170mm of space I'd look for the longer style lipos since they sit a bit lower than a normal length pack of similar capacity. I'd probably go for the 5000mah 25c enerland cells (flightpower, poyquest and a few other brands that I forgot), but you should be able to loose some capacity if you want, since 5000mah 25c packs can do 125 amps, and you'll be using 2 in parallel...good luck trying to put 250 amps at 4s to the ground
