TZ85s will run wheelchair motors quite nicely, you'd be looking at ~£60 from Hobbyking plus the gear to hack them yourself (another £10-15 depending how fancy you want to be). I know Harry's been testing something called a Cytron too, from Malaysia - that's apparently not unreasonably priced at £120ish and has the advantages of dual channel control and therefore internal mixing.

TANGENT ALERT: @Mario specifically, I've heard a lot mentioned about the Mythras speedos but never found much information on them - what are/were they?