i know nothing of vintage... but if your serious, first decided what your going to "use it for"... or are you just wanting a project? are going to do an original restoration, custom, cruiser or racer? a kitted out monster? a bit of all of them mixed together? depending on purpose, you'll have to decide on frame size, wheel size, engine size... also check on the availability of replacement parts on a particular series... some small frames are missing battery and engine doors that are impossible to find, the rally 180/200 are sexy machines but the cowls are worth more than the scooter now... if you can find cowls! vba and vbb gas tanks? forget about it! of course the P series have the best availability of parts and optional after market bits...
truthfully and i've never done this type of thing, i would suggest; finding the frame you want and then look for engine cases and build it the way want it. i prefer stock or semi-stock motors(sito-plus, bigger carb...) and a flashy paint job. i don't want an "original" restoration of a stock bike... i also don't care about keeping original parts... like a GL head set on a Super or cutting and fitting a P glove box for a Sprint... that type of stuff. buying a couple of scoots and parting one out... it's the cheapest way to do it and the funnest... it's definitely a long time project!
even though i don't recommend buying one, the [redacted] web site has a lot of great pics of several different vintage bikes and paint jobs.
have a good and clean work space and the right tools can make it way more enjoyable...
good luck and let me know if i can offer any help.