Nice score. The 645s are good machines. I had one myself until recently, when I upgraded to a Model 847. Mine was probably in worse shape than yours when I got it. Here’s the thread about getting mine cleaned up:
Thread 'Index Model 645 Mill'
https://www.hobby-machinist.com/threads/index-model-645-mill.49582/
You’ve got the table power feed, which is nice. If you’re lucky, it has an R8 spindle taper. They also made them with B&S #7 tapers, for which tooling isn’t as plentiful these days, but I think you can still get new collet sets.
Keep us posted on your progress!