I just cracked my frame, and want to re-use pretty much all the existing parts. I'm running knobby 700x35 tires and so I believe I'd need a 700c or 29er frame. I'm going to be carrying a good deal of weight on the rear rack and I want a bike that's bombproof. I'm looking to get something used to save money, and haven't really found anything on eBay or craigslist. Everything seems to be super expensive. Does anyone have any advice?

I'm considering anything from full suspension bikes to old steel road bikes. Durability (and a large size frame) is really all I care about.