Neilin,
I've shot my M51 mabey 4 or 5 times a year, 100 rounds each time, for mabey 7 or 8 years now. The only problem was a broken extractor which I was able to replace new (Brownells or Gun Parts, I don't remember now). No big deal. The gun shoots very well and magazines ARE available but you'll have to look and they go for mabey $30. Not that you need that many.
Breakdown is tricky as is locking the slide back but if you do a Net search there are instructions out there with illustrations to show you how to fieldstrip the gun.
By the way, the M51 can be made to shoot hollow points.... just not the first round. Use military ball for the first round and then hollow point and it should work (at least it does with the original magazine). Colt 380 magazines will fit in the M51 but you may have to play with the feed lips to make them work reliably.