HI I have built many old bikes into Motor Bicycles, and from the look of that frame it's NOT done-for. Just check really close for cracks, when the metal is all clean, a "springer" of any sort, or even a hydrolic front end will reduce the pounding that the frame would take from riding.
All this said you are NOT going to be a curb-jumping, off-road maniac are you? If so please pass by everything that is vintage and special, and buy nothing but cheap, new replaceable stuff so the good stuff will be saved for normal riding.
I can show you some cool ones I built if you like,
Police stations sometimes will sell impounded bikes and the city dumps can be a source of bikes, can't do that where i live found out after the fact you can't take stuff out only allowed to put stuff in. Alley ways a good source but you should ask before taking. Wreaking yards a good source to I found one close to where I work they use bikes as a little extra material for scrap weight so I have to be quick cause the wreckers trash the bikes and unless I ask for a certian type they will smash them as quick as they get them.
Yard sales another good source. Don't forget Wally world they have sales and you can get a real good deal sometimes but then again cheap made bikes or they can be.
I started out just rebuilding my kids old mountain bike huffy built in 1989. I was either going to rebuild it or throw it in the dumpster. After fixing it up and enjoying riding it around I thought it needs an engine and its been all down hill from there.