I would look at Rtings.com, some specific TVs have better upscaling for 720p content than others. Vizio, for instance, is known for poor 720p upscaling in their P-series models, relative to the competition. All of this may change in 2019 of course, but until units are in reviewers' hands and tested, we're all speculating on which will perform better. 1080p upscaling tends to be quite good in all popular 4K TVs nowadays, especially at your price point.
Note that if your source input is at 720p, and you're using an AVR receiver, the AVR receiver may be able to do the upscaling for you, which may be better quality than what your TV is able to do. But this adds a processing step which will result in some small amount of input lag.
Different TVs have different levels of input lag for different resolution inputs. For instance input lag for a 720p source vs. a 4K source may differ. Again, I recommend checking RTings.com. For instance, the very popular Sony X900F has a higher input lag relative to the competition, although some will notice it and some won't. Depends on what games you're playing and what your expectations are. Safest route, of course, is to go with a TV that minimizes input lag.
I would also avoid OLEDs as they have burn-in risk associated with HUDs in-game. Some people will tell you this is an overblown issue but it is a known and documented problem, and there are plenty of examples you can find in this forum.