The BEST way, with the LEAST hassle and risk, is find one near you in your price range, and inspect it yourself.

Shy of that, your decision is hanging in the balance of someone who is trying to make a profit on selling a car, or someone who may not know squat about the model or its quirks and weak points, if it's a long-distance purchase. Then there's the hassle of shipping, and hoping that when they tighten the car down, they don't hook it up by the tierods or the bumpers, which happens all too often...

In a 3.5, look at:
-Engine tone and running at warm idle (no missing)
-Load test (no missing, no smoke, no tranny slipping) - floor it at 75 or so, it should downshift and you should be pushed back in your seat.
-Smoke, leaks, noises
-Handling - no wandering or vibrations at higher speeds, especially 80MPH+

If possible, yank the right side valve cover (4 10mm bolts) and examine the camshaft, if it's grooved or especially if the lobes are flat, that means it needs new cams and rockers, and possibly more engine work. (This is a result of overlooking a simple maintinance issue, a point where something cheap causes expensive damage)

Smoke at idle (especially at startup), or under acceleration briefly after startup, (or even on decelleration from a hard run) means you need new valve stem seals - a PITA, but not too costly. Don't be thrown back by this smoke.

Compression test should be effective enough for detecting engine troubles.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k