Look to a genuine Montague as a donor instead. These have been built into great roadies. That one is going to be very heavy as you point out; by the time you get it light and nimble enough to make it into the peloton (if ever) you are going to be a good wad of cash lighter. Plus it has rear suspension which is a total gimmick, sapping energy while giving little comfort. That BSO is 15kg at least. You would be replacing every single component. So while the starting point price is low, the end point would be higher, with a good chance of not being satisfied in the end.