leader frames are fine but by the time you've built them up you've spent more than a BD bike and depending on the components you buy, more than a better prebuilt bike like a langster or felt.

i was just pulling random numbers, but if you really are trying to lose 20 lbs i'd suggest incorporating bicycling into a more comprehensive fitness schedule/plan. you want a bike that will be fit properly so i'd suggest going to a good local store that will actually take care of you and fit you properly.

if you get a 720tr or an SST-AL or a Madison you're going to need to do a lot of component selection and build fitting that you might not be used to coming from mtbs.

and if your knees are toast i think it would be worth having a geared road bike so you can do inclines without causing damage. but if you want SS, do it.