When I buy a used brick, I have my dealer run it through their certified car check. It is way too expensive to repair a Volvo that has problems.

I get a nice two page report - I think that they can tell if an ant farted in the trunk.

I pay 1 hour's labor - add an hour if you want a compression check. Probably a good idea in your case.

I f the car looks good during the test, I usually tell them to put new plugs in and save the old ones. I use them as spares, besides I like to look at them.

An interesting story -

I was buying a used car off this dealer and before our final negotiations, I paid for a certified check.

The next day the salesman called me and offered to pay for half the cost because the service department would not let him look at the results!
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks