I have the USP, the 220ST, and the P99 .40.
All three are very accurate, but I seem to do best with the HK. Recoil is about the same between the HK and the Walther, IMO, but the 220ST is so heavy that it dampens the .45 recoil down to the level of my 9mm Glock. I favor the grip angle on the HK because it is the most natural for me.
If you want a compact, concealable pistol, go with the P99. It has more recoil than any other .40 I've used, but it is also very controllable due to the ergonomic grip. I've never had any issues with this gun yet. I do like the magazine release on the P99 because it is ambidextrous. I just use my trigger finger to dump the mags. It has the same release as the USP.
The P220ST has very mild recoil for a .45. The trigger on this pistol is perfect; it is smooth and light. The only down side to this gun is the size vs. magazine capacity. The grip is the same size as that on my HK, but it doesn't use a double-stack mag. It could just be the factory rubber grips that make it fat. I've never had any problems with this pistol either.
Finally, the HK, which is my personal favorite. Recoil is not bad, even though it is a light weight pistol. Honestly, I think the P99 recoils more than the USP. DA trigger pull is not one of the HK's strong points...it sucks compared to the SIG or Walther. The good thing is that the HK can be carried cocked and locked in SA mode, which is very light and smooth like the SIG. I've never had any problems with this gun either (strange, huh?!?!?
) Again, I favor the mag release on this gun.
In order to get a suppressor, you do not need a C3 license. However, you will need to fill out an ATF Form 4 and have it signed by your local sheriff, police chief, or state attorney in your district. After that, the form goes to the ATF with a $200 tax and will be approved in 2-6 months (takes a while, I know), at which point you can go get your suppressor at the shop. If you actually want to deal in C3 weapons, you need to be a Class 3 SOT (Special Occupational Taxpayer). If you go this route, make sure you are not just getting the license to buy for your collection. You need to be able to show that you are a legit dealer, otherwise ATF and/or the IRS can come down on you hard.
The best of the three guns to suppress is the HK Tactical. The barrel is already threaded and properly weighted to accept most of the suppressors out there. It will be less of a headache than trying to get either of the other two to work correctly. Also, the .45 is subsonic, which makes it easier to suppress. The .40 can be suppressed, but you will hear a ballistic crack as it breaks the speed of sound. The "crack" sound cannot be suppressed.