Yeah, speed controllers get expensive once you get above 500W motors (about the limit of a TZ85). Wheelchair and scooter motors can be used, but as a rough idea, most heavies use motors with about 1200W peak power (Bosch 750s).

If it's just a test-bed bot though, wheelchair motors and Botbitz TZ85s will be fine. These motors would probably also work ok http://www.ebay.co.uk/itm/ZY1020-Ele...kAAOSwz2lXDMyJ

A better but more expensive ESC are these: http://www.robotpower.com/products/vyper_info.html

Quite a few heavies use these and they let you run motors up to around 1500W