Winkey or Winkeyless? This is just a question of aesthetics, but WKL kits usually end up being more desirable down the road if you ever decide to sell it.
What firmware do you want to use? Again, a matter of personal preference, but the hierarchy is generally: QMK = BMC > JigOn > O2D (duck boards) > Aikon (older TKLs, mainly OTDs)
What typing angle do you prefer? 8 degrees is pretty much the standard, but you see other options pop up now and then.
Who makes it? Is is some random running their first GB, or is it someone well established like LZ or duck? Are you willing to take the risk on quality of a new runner?
Is the PCB standard? If it's not A.87, I'm not buying it.
that's the basics, there's always little things i look for that i can't think of right now