That has a type of bolt on hanger not used these days but low end bikes. Given what you're planning to put into is, I'd work up a total estimate and compare it to what you can buy in a new bike. Of course if you have some of the stuff already that's a different story. Otherwise if you sink lots of dough into this you'll have an expensive machine, but still not great bike.

Depending on how hard the hanger is, you might be able to drill and tap it to a 10x1 thread to accept a modern derailleur (test the hanger with a file before you kill the drill). You might also have to do some file work to get the stop tab to engage at the right angle.