4S or 3S is fine. Here is a 3S lipo for the a little less: http://www.ebay.co.uk/itm/Gens-Ace-2...item2a3faec8ae

Here are two 4S options http://www.ebay.co.uk/itm/Gens-Ace-1...item27e5f07f02

http://www.ebay.co.uk/itm/Gens-Ace-3...item27e5f0bd84