A friend of mine just bought a similar setup.
http://www.bintel.com.au/Accessories...oductview.aspx
http://www.bintel.com.au/Accessories...oductview.aspx
Then he got a 7Ah battery from Jaycar for about $40 plus a lead with clips and a lighter socket for a few $ extra.
Then he got a battery charger from Super Cheap Auto for about $50.
Unless you want to DIY I really don't think you could do it much cheaper!
