There is hope. I have an older centurion super le mans that I am doing the same thing. I happened upon a Thrift store Univega viva sport that is now a single speed because I jacked the 600 arabesque drivetrain. I also responded to a cl add a few weeks ago from a guy that was moving and needed to get rid of some stuff. I got a pair of Wolbers laced to 105 hubs, 105 derailleurs, and cassette, And a 1994 gt mtb for 1 whole dollar. It just takes time and a little luck. The only things I plan on buying new are tires, tubes,seat, and bar tape. Your build is totally do-able in your price range. You can also build it somewhat to your liking while still keeping an eye open for smokin deals on your ideal parts. Most of the fun I have recently discovered with vintage is the hunt anyway