Okay, I'm a bit confused here. Usually, people ask about a specific version because they own it. What is it about 12.5 that has you stuck on that version? Buying a used license is fairly simple, as long as you deal with a reputable seller so you know they will be forthcoming with the required iLok asset (assuming you have an iLok account). Needless to say(but I will anyway), if someone offers to sell you Pro Tools that doesn't need an iLok, steer clear of that. The only reason I can see for stopping at 12.5 HD is to save the $299 cost of a Digilink license(required with 12.6 and up), but as that's a one-time purchase, it seems a shame to sacrifice the multitude of added features in the newer versions. Having said that, the choice is still yours and I would start your search with specific dealers like Alto Music, GSharp, Dale Pro, Parsons Audio, etc.(Alto and GSharp are both big eBay sellers too).
Just to recap, here's what you need:
1-HD or Ultimate software(can't use the native card or 192 without the proper software)
2-an iLok account and a 2nd or 3rd gen iLok
3-a Digilink license if you go newer than 12.5 HD software
4-an open PCIe slot in the computer(BTW, the HDN card can often be used in a longer PCIe slot)
5-Pro Tools software(which usually requires an Avid account and registering the software, so if you don't have that, make sure the seller can provide you with a way to download the installer)
