I highly recommend Rory's kits:
http://www.fightingrobots.co.uk/thre...antweight-kits

They are by far the easiest way to build an antweight and are really lightweight and compact so make it much easier to get the antwieght in weight/ in the size limit.

This forum is more directed towards antweights and you'll find more help/information there:
http://robotwars101.org/forum/index.php