95 concorde 3.5

Valve cover gaskets are an easy job on that engine- clean the intake plenum while its off, check vacuum lines for cracking and replace as needed, etc. Unscrew the big nuts off the top of the 6 spark plug tubes and leave the tubes in place- makes life much easier. At 117k I'd replace the timing belt if I had any doubt about when it was last replaced.

As for "can you get another 20k?" well, my wife's 93 has 250k on it, so you should be able to get in the neighborhood of another 120k out of it assuming you maintain it and fix things when they break. If you just "turn the key and go until it quits" (IOW abuse it), you might get another 20k. Or another 10 miles, who knows.

If otherwise the engine, transmission and body are in good condition I'd say YES. You are at the point where you are trading maintenance for much higher depreciation costs. Your mileage is not that high for that car. Since you say you like the car, why not keep up it's maintenance.

Before replacing the valve cover gaskets you might try tightening up the valve cover bolts just a bit.