I have a 5.5" stainless Ruger 22/45 MKIII. It's easily my favorite range gun right now. My range is less than 5 minutes away, and oftentimes I'll just grab the Ruger, a bulk pack of Federal's, and a grocery bag of empty soda cans. It makes for a great afternoon. The only modification I've done to my 22/45 was to remove the magazine disconnect and replace it with a spacer. Disassembly and reassembly are not as difficult as some have made it out to be (for me, anyway).
Personally, I think you'd be happy with any of them, but I particularly like the 22/45 and the Buckmark, mostly because of the grip angle. The angle of a steel frame Ruger (MKII, MKIII) is steeper, kind of like a Glock. I shoot them fine I guess, but I prefer a shallow angle.
I also like the styling of the Ruger and Browning better than S&W and Beretta, and I'm not a fan of the little guns from Walther and Sig, although I've never shot any of those, just held them in the shop.
The longer barrel lengths offer increased velocity and a longer sight radius, although the inherent accuracy of the guns will be about the same. Get a long enough barrel for a good sight radius, but small enough that it balances well in your hand and won't be awkward to take to the range. I like a barrel length of around 5", but you might be different.
If you can find a 22/45 MKII at a reasonable price, I'd go for that, otherwise I think you'll be happy with any of the offerings from Ruger or Browning. Based on hearsay, I'd recommend a full size pistol rather than a small one (P22, Mosquito, etc.). Apparently they're much more reliable.
BTW, a .22 pistol is a great idea.