Keep this in mind: Motorcycle replacement parts are typically marked up at least 300% over the price of the original motorcycle and manufacturers typically support replacement parts for no more then 7 years post model production. The longer those OEM parts sit on a shelf somewhere as inventory the more they have the price marked up. This is how a 5 year old plastic fender sticker ends up costing a hundred bucks or becomes totally unavailable.
There are a Lot of consumable parts on a motorcycle. If you buy a worn out 4-cylinder non-running motorcycle and throw mail order parts at it, all you will do is constantly sink money and time into something that you can rarely or possibly never ride. Too many of those un-finished, over-priced bobber-chopper projects are on the used market now.
You're young! If you really want a nice old motorcycle to actually ride, what you do is buy a really Nice new single cylinder motorcycle, ride it solo until it's old and now you have a nice old, ridable, well sorted motorcycle that you know still runs like it ran when it was purchased new. With a single you will be done fixing and out riding in 1/3 of the time, you will need to buy fewer bits to fix or maintain it and encounter fewer mechanical complexities to deal with.