You have to crank the engine with the electric starter and a stout battery to spin it fast and consistently enough to get a valid compression test. Pull all the plugs, and disconnect the coil primary circuit by pulling off the brown wire and making sure it doesn't touch anything. Use a screw-in gauge if possible. You are looking for all cylinders to be fairly close to each other (say 15 psi max range) rather than the actual numbers, which are dependent on technique, (throttle open or closed, engine warm or not, etc).

KBB value for one of these in "good" condition around here is $2900. Your deal looks good - maybe too good?

If there is any doubt about the car, getting a mechanic's opinion would be very advisable - the cost to do repairs could easily exceed your purchase price.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-940Sedan B230FD)