I thoroughly encourage that idea. I love putting Fat Franks on old mountain bike frames to make bomber commuters. I just finished this '91 Trek 970 up the other day:

'91 Trek 970 - Album on Imgur
Having done about 3 like this, I might recommend going with the slightly smaller Schwalbe Big Bens to guarantee fitting fenders up front. The fork crown is where I've found the least clearance with balloon tires.

Also, as Gugie said, depending on where you live you can definitely get by with less than 300 upfront. Patience, and a regular review of CList, will be rewarding.