If you don't have a lot of machanical knowledge concerning bikes I would definiatly
not make your first project frame up. For the guy down the street that would certianly be the best way to go but where's he going to be at 1 o-clock in the morning when you're beating your head against the wall because your final sprocket doesn't line up with the drive sprocket. that's the kind of thing that can sour you on motorcycles all together.
I wanted to do exactly what you're talking about so I bought myself a decent
'75 Honda cl360 (click to see my project) for just $300 (ironicly $74 less than I spent on my helmet) and with a total investment that probably figures to about $400 - $450 and a good amount of personal labor I'm on my way to a pretty cool cafe.
This bike is the first motorcycle, the first air cooled engine, and the first carberrated engine I've ever worked on, and I've learned so much. There's a lot of things I would and will do different on my next project, but I learned all that with a $400 investment instead of $4000 one.
Also as far as the garage goes, make sure you have a good place to work. My garage originally had one incandescant bulb stressed with the obligation of lighting the whole place, I couldn't imagine being able to get anything done in there so I went out to Home Depot and loaded up on florescants and put them all over.