Well I havent built one yet, but from the research I have done I would say that twin downtubes up front is important, for mounting the batteries.
I think any of the older streetbikes will work fine. Ive got a soft spot for the gixxer 750 so thats what I am looking for.
Any of the older standards should have plenty of room. CB750, Eddie Lawson Z1000
I think the trick is to go to your local wreckers & find a decent roller cheap.