I need to know as fast as possible a normal range of compression I should get when testing my engine.

I heard some engines were tested at 150-160, but how low can I go?

I *think* you can go a bit lower than that with an 8.5:1 motor and still be okay. Mine was at 120 across the board and as far as we know it was fine... as far as the compression was concerned.

Is there anything else I should check before buying it?

If you have access to a leakdown tester, do that. Other than that, check the turbo shaft and make sure it's not loose or wobbly. If you can move it a very tiny bit that's ok, but it shouldn't move very much at all. If it's moving around, you're in for a turbo rebuild. Maybe pull the valve cover off and look for irregularities, see if there's any sludgy buildup, make sure the cam lobes aren't damaged.