well thats really upto you drills arent expensive but if you have 4 rather than 2 thats more weight more wheels bigger robot more weight again but probably better control

when you say how many battery packs that rather depends on the battery packs themeselves

i would personally go for either 2x 8.4volt packs or 2x 7.2volt packs with 2500mah + i would say per pack

alex