For the 16” crowd, Ruger and Henry make options that take different magazines… one option being Glock. Colt’s 9mm is good, too… just make sure you use good magazines (all of mine are Metalform).
But if you can do a braced pistol… opens up A LOT of options.
Can build an AR. Colt pattern or another pistol magazine adapter. I can vouch for Stern, as I have their Glock setup in my suppressed SBR. Stick with the blowback options. CMMG’s radial delayed blowback is nice… but higher costs. I tried the Maxim recoil assembly, worked but in the end… I went back to standard blowback just for simplicity. Either pistol or rifle, if you go with a 9mm AR, take a look at this site for info…
https://blowback9.wordpress.com/
There are different chassis options for a few pistols. I just got a Flux Raider X in FDE. With first responder discount, I got it for $500 shipped. Should be here tomorrow… bought on Friday and was shipped within the hour. Another $200ish for a FCU… and then looking for an M17 slide assembly. If I just got a surplused P320… would be way under $1,000 with some funds for a red dot and magazines. But this is going to be a project for me… eventually Form 1ing it. But if state law allows (mine does, but stock is more better), the brace can work fine.
Beretta CX4s are a good option. I have the SUB-2k Gen 2… and very happy with it. It’s the Glock 19 version, which I try to use as a platform most of the time since I’m issued one. No background with the S&W… but I don’t have a 9mm M&P, so moot point for me (I hesitated with the SIG because I will not likely invest heavily into the P320… the Flux setup, but might get an M18 kit).
H&K clones… might still be able to find MKEs in that price range. My PTR Classic, also SBRed, was in the $1,600 range.