If you're moving into Heavies, I'd advise going for LiFePO4 batteries. They're pretty much as safe as NiMH, but lighter and easily available. Quite a few heavies use them these days. You need about 8Ah for drive. You'd want two of these in series: http://www.hobbyking.co.uk/hobbyking...arehouse_.html

Otherwise, SLA batteries are good if you think your robot's going to end up quite light, and they're nice and cheap. You need quite a large amount of capacity compared to other battery types, as they only produce good current when fully charged (Nuts used 14Ah at 24V). Two of these would do nicely: http://www.ebay.co.uk/itm/CSB-EVX121...gAAOxy2YtRr168