Staingate isn't happening to everyone, just a small percentage of users, though the cause is still not known. By and large you should be okay on that.
As for whether a 2012 MBP would be fine for Lightroom, absolutely. It's more than capable of running it. The only question is whether the increased resolution on a retina display is worth the upgrade for you. If the lower resolution is fine, or if you think you might be attaching an external display a lot of the time to your laptop, then the 2012 MBP will be fine for your needs.
If you have an Apple Store nearby or a Best Buy selling Macs, you might want to compare screens. I doubt they have 2012 MBPs lying around, but they most likely have 21" iMacs and MacBook Airs that you can compare against a retina MacBook Pro, to see if the difference is that big a deal to you.