I realize that "luxury" and "under $20k" don't quite go together, but recognise the desire as stated even though the "baby" cars really aren't the same as the "senior" models when they give up wheelbase/road weight/big power.
Cost of maintenance/repairs/insurance is what I would be looking at over the projected second-owner lifespan. Edmunds (see their site map) has an excellent feature for helping with this and I recommend starting from there.
"Luxury" cars are only that . . . if the owner can afford to keep them up. Nothing so sorry to see as a once-nice car on ragged tires, sagging suspension, dirty undercarriage, or with poorly-done body repairs. It shouts, "I'm too dumb to be trusted with money", and achieves only the impression on others exactly opposite the one intended.
With that said, I used to love to pull up to any of the baby "name cars" in my old Chevy or Chrysler and challenge them to any kind of race they chose. Win, [rarely] lose or draw, none of them could come close to matching the little money I had invested (did my own work except body/interior), nor did their cars have any features I could have added if I'd wanted to. Shoot, once you've got PS, PDB, AC, AT, cruise, power locks and windows . . what is there? You can't add wheelbase, etc.
Running away from them, especially in that Chrysler, was always a pleasent way to "shorten" the commute.
Find the one you'll enjoy spending money and time on, for any "luxury" car always needs both to be at its best.