The car came with one. Dont suppose it is still there?
The American tool makers finally got around to making 18mm plug sockets. I would get a spark plug specific socket with a built in universal joint. It will make life easier getting front bank plugs out.
160 PSI is a good reference point. The best use for the compression test is to look for significant variances in pressures from cylinder to cylinder or from bank to bank as opposed to the peak pressure.
A variance of up to 15psi is generally "acceptable", anything more indicates a real problem.
If you want to verify that a cylinder is in good condition a "leakage tester" with compressed air is a much more accurate tool, it will also tell you where the failure is: piston rings, exhaust or intake valves.