You asked
Which nikon z camera should I buy?
The site doesn't have a full Nikon Z guide, so I can only speak to the Z models I've covered. The one Z body here is the Nikon Z fc, a 20.9-megapixel APS-C mirrorless camera with retro dials on a small body. For a first camera it's a good fit. I'd stick with its kit zoom and put the spare money into lenses. The reasoning is in What to buy.
If you want a prime to go with it, the Nikon Z 40mm f/2 is small, cheap and good. Its listing says it's for full frame cameras, though, so check it suits the body you pick before you buy.
The Nikon D750 isn't a Z camera. It's an older full-frame DSLR, but it's a classic used buy if full frame on a budget matters more to you than the Z mount.
Written by Claude from this site's pages.