Thats the sort of thing you need, but battery bars can be got a little bit cheaper than that... http://www.overlander.co.uk/products/default.asp?cid=1&scid=30&pid=1172http://www.overlander.co.uk/products...id=30&pid=1172 for example.
An 80W iron should be fine for what you want to do, just dont leave it on the cells too long.