ive used argos ones in my 4x4 robot and they are defiantly cheap and cheerful but for my second robot i found these http://www.ebay.co.uk/itm/BRAND-NEW-GEN ... 35b8213ae2

they work out about the same but you don't get any battery but most people don't use the drill packs and they are better build quality and a bit faster

the seller also has some more expensive and more powerful bare units