The higher version hardware would be preferred, but don't get hung up on it. You won't be doing trunking. Find one that looks good, is a good price, and the seller is reputable. For ham use, any of them will work.
For GMRS, none of the TK-5310 models have the required Part 95 certification.
The K2/K3 are the high split models, 450-520. Unless you really need to listen to something up in the T band, you'd be better from a ham standpoint with the K5/K6/K7. Those are 380-470MHz. While you can often take the high split models into the 70cm band, you'll likely have issues with performance down near the bottom of the ham bands.
You can have more than 16 channels in a zone, but the rotary selector on the top will only let you choose 16. You can program front panel buttons to do the channel select and have access to more.
KPG-95, I'm not sure what the latest release is, but I know I'm using 8.10, and it's from 2014. Not sure if there are newer versions.
No tricks to getting KPG-95, the dealer will happily sell it to you.