Once you focus on reliability during the second 100k, you're pushing yourself into a Lexus or maybe an Acura. Among luxury sedans, nothing beats the Lexus LS for long-term durability. The ES might be close for the first 200k, but after that the LS likely pulls ahead. The IS and GS has been spottier, but still far less likely to become troublesome than one of the Germans. The current generation IS and GS handle better than earlier ones, but they're far too new to find for $20k.
Over at Acura, the RL might hold up better over the long-term than the TL. It has largely slipped under the radar, so you should be able to get one that isn't terribly old within your budget (possibly an issue with the Lexus LS). 2008 is the last year for one without the ugly grille, just outside your cutoff.
The Infiniti M could also be a contender. Just remember that 2011 was the first year of the current design. Withany model the first year of a redesign is often at least a little less reliable than later ones.
The Genesis is too new to say how they'll do with a lot of miles, as the 2009s now average about 60k miles. Its record so far has been iffy--though mostly electrical things like the nav system and motors for the tilt/telescope of the steering wheel. If you do get one, those without the Technology Package avoids these common problem areas.
The upshot: if you want the most reliable luxury car for $20k, you'll likely end up with another ES, as any LS that's the same price will be much older. Newer is almost always more reliable, so at a given price point there's a trade-off between the inherent reliability of the model and its age.But it's worth checking out the IS, GS, TL, RL, and M to see if any advantage in how they drive over the ES is worth somewhat less long-term reliability.