<$15k will get you an awesome cushy car.
Yes, Buick seats are really are too squashy, even for me.
IMO go all the way and get a Lincoln or Cadillac.
The depreciation curve on these cars is unbelievably steep. You can get a perfect one, only 5 years old within your budget. An 8-year old one can be had for about $8k or less. People often will ask more, but its rare they get their asking price.
You may have to look around for one that has been maintained properly and is in excellent condition... because excellent condition does not increase resale value much.
The Caddys seem to perform better.... the Deville is better for a traditional luxury feel and the Seville STS (not SLS) is smaller and sportier. The Northstar V8's are really fast, yet get respectible mileage.
The only problem with the Caddy is the Northstar has a cylinder head bolt problem, which tends to cause blown head gaskets in about half the engines out there.
The fix is to install a "TimeSert" kit. This is about $2500. But, everyone knows this, so used Caddys are priced accordingly. I have seen the odd-perfect Caddy with blown HG's going for $2k. Repair it for 2.5K and you have a car worth $8k.
The Lincolns are more reliable than the Caddy yet still cheap to buy. The Town Car is like a rock.... this is why all airport limos are Town Cars, they seem to last forever, although I like the Caddy independent rear suspension more.
The Touring Sedan package is the hot set-up - like a Lincoln police car - but they are pretty rare.
Otherwise, the smaller Lincoln LS is a great car, very good handling with excellent all-around performance. . The V8 model is a bit faster and more reliable than the V6.
Beware of the 'active suspension' options. Shock and struts get very expensive to replace, although you can substitute 'passive' regular shocks and struts for much less money.
Finally, take a look for a used late-90's Eldorado or, especially a Lincoln Mark VIII. They, especially the Lincoln were astounding cars...... the trick is, finding a really good one because even the newest ones are 10 years old. But they are still cheap to buy, are awesome and will only increase in value as they become more collectible.