Another important point was brought up and that is the sights. Most pistols will not have sights that you can see with the suppressor mounted. Some people say this isn't an issue. You just shoot with both eyes open and you create sort of an optical illusion that you are looking through the sights. I have tried this and it doesn't work for me. I want and need sights to shoot accurately. When I bought my 9mm suppressor, I bought threaded barrels for several of my 9mm guns: Glock 34, Glock 17, Glock 26, and Browning Hi-Powers. I also put larger sights on the Glock 34 that were sold for use on suppressed pistols. In the end, I NEVER use the suppressor on any gun other than the Glock 34 because it is the only gun that I can see the sights.
FWIW: I also have an HK Mk.23 with an SWR HEMS suppressor. I know you said that the HKs were out of your price range, but there are a couple things to consider: the HK came with a threaded barrel. So, you don't have the added expense of buying a threaded barrel or having a barrel threaded. Second, the HK came with sights that were designed to be used with a suppressor, so you don't have to buy new sights. Again, I know you said you didn't want to spend the money for HK. But when you are making your decision on a pistol: take into consideration what you might spend to turn it into a dedicated suppressor platform. If you need to thread a barrel (or buy another barrel that is already threaded) and you intend to add higher sights: factor this cost into the project and weigh the advantages/disadvantages of spending more initally to buy a pistol that already has this stuff.
9mm vs. 45 ACP
With suppressors, the 9mm will be quieter than the .45 ACP. As I understand it, the smaller the hole in the end of the can, the quieter it will be. I dont' have the latest thing out in a .45 ACP suppressor. Mine is a few years old. They have allegedly improved somewhat over the last few years so maybe mine isn't a good example. But, my 9mm can, shot dry, is about as loud as my .45 ACP can shot wet. Shot dry, the .45 ACP can is kind of a disappointment. It is suppressed, it isn't nearly as loud as a .45 without the suppressor, but it isn't all that impressive. When shot wet, it is pretty quiet. On the other hand, wetting the 9mm can makes it quieter than an air rifle by a wide margin.
If I could only own one centerfire pistol suppressor, it would be an 9mm without a doubt.
One more thing that you might want to consider: if you go with Glock, threaded drop-in barrels are readily available. And, they arn't expensive. YMMV with other guns. You don't have to send the barrel away to be threaded. You don't have to wait to get it back. You don't have to pay a lot. I have purchased a threaded barrel from Bar-Sto (as an example). The barrel was VERY expensive and took many months to get. I have also sent a lot of rifles away to have the barrels threaded be a gunsmith. The cost of shipping (not an issue for you since you aren't shipping a rifle but........) was amazing and again there was a several month wait to get it back. With Glock, I just bought a couple of my threaded barrels at a gun show and they drop right in (I had to fit the Bar-Sto barrel). Instant gratification.
Stuff this much fun should be illegal!
