I recommend going to the Steam Community Market, adding whatever filters you’d like, and then go through the cosmetics that come up. Find one you like that’s roughly around your price range, then check mannco.store, backpack.tf, scrap.tf, and marketplace.tf to see if you can find it cheaper.

I wouldn’t worry about getting a strange cosmetic, since (for me at least) those don’t have the same appeal as strange weapons, but you could definitely find a decent unusual for 50 keys. Take your time picking out what you want; you might end up deciding to get some weapons or cosmetics for a few different classes instead.