I'm in much the same boat, so I'll share what I considered and then ended up (so far) doing and why. Those with far more experience can tell you if I've done anything right or just run off the rails.
I wanted to get something that would be fun to shoot, but also be a decent enough investment not to be a big money loser. (I have bought and sold quite a few guns without managing to ever do more than break even, and seldom that well.) If I could have anything within reason, it would be a 1928A1 Thompson SMG. But they are very pricey, and I think you need to know more than I do to commit $10k or more to something that might turn out to be not quite right and worth a couple of grand or more less than you thought. Plus, I don't know if I'd feel comfortable shooting something that expensive, so it might end up just sitting in the safe, which does nothing for the fun factor. At the other end are MAC-10's and 11's. I thought very seriously about a MAC-10 in .45ACP. But there seem to be so many around that it's hard to see them appreciating much. They also have an extremely high rate of fire; looking at a carbine model, all I could think about was how fast the barrel would heat up with all that money pouring out the end. I like the look and feel of the Reising M-50. They are affordable, but magazines seem to be getting scarce. I wonder if anyone has really tricked one out? I might end up with one of those yet. I shot an H&K MP-5, and it's a great gun. But very expensive and almost too nice. Sort of like the difference between a shovelhead Harley with drag pipes and a smooooth BMW; a matter of taste, I guess, but I'll take the Harley. Also, I don't know enough yet to have any feel for how the market thins out above $8,000, but I'd guess demand drops off up there.
So, I've ended up buying two Uzi SMG's. Both are registered receiver guns, one is a used (98%) IMI, the other a new Group Industries gun from Vector Arms. Neither has been delivered yet. My plan is to shoot the IMI and leave the other one NIB. Or, maybe I'll do a .45ACP conversion on one and shoot them both. Or, maybe I'll decide I like one much better and sell the other one. Dunno, have to see. But I figured Uzi's are a popular and proven gun that shoots cheap ammo and is in a price range where demand should stay strong, especially once the Vector inventory is sold off.
FWIW, which might not be much.
Byron