You'll have to arrive at what you like or don't like, what you need or don't need, how much you think is ok to pay or too much.
What I can offer is be sure you give the car a thorough check out.
Arrange to visit the car ideally with the engine dead cold and get a test ride then a test drive.
The route wants to cover 15 miles or so and give you a chance to experience as a passenger then as a driver the car as you intend to use it.
Open the trunk and engine compartment lid, turn off the A/C and turn on the key and be sure all warning lights come on, especially the CEL, and all go off as soon as the engine starts or very very shortly after it runs.
Carefully listen to the engine for any abnormal or scary noises. Let the engine idle and you walk around the car ostensibly to check the body/paint (which you really want to check) but also to let the engine idle to near warmed up all time with an ear tuned to the engine.
After 10 minutes or so then have the seller take you on a test ride.
Once the engine is warmed up have the driver do a hard acceleration run from near idle up through at least one gear, say from 1st on into 2nd and up to redline before backing off and upshifting to a higher gear. The engine should pull smoothly and strongly from idle to redline and after the shift resume its pulling. Then once the drama is over the engine should settle into a nice smooth idle like nothing happened.
Back at the starting point then you take the car out and drive the car over the same route, driving the car the same way.
Even after your test drive the engine should not manifest any abnormal or scary noises.
If after the test ride and drive you still like the car then you give it a thorough used car check out. Get a copy of the CPO check list and use that as a guide doing what you can do working from the list. Assume nothing works until you verify it does work.
If after all of the above you still like the car and believe you can buy it for an agreeable sum then have the car PPI'd. This inspection will look for things you might have missed: body/paint work; signs of repairs under the car; leaks, and so on. The DME run time and over rev counters will be read.
Try to avoid falling in love with the car until you are sure it is the car for you and is worth owning.
Remember there is always another car.
Happy shopping!