Buying a diesel compression tester? Plse advise.

I was thinking about getting a compression tester for my old M-B diesel. That way I can keep a better watch on what is going on with the engine, if the engine is shot and needing overhaul, etc. Its the '85 5-cyl 300 SD engine, which is supposed to have the valves adjusted regularly. Autozone has compression testers for *gas* engines, and they are only around $20.00. But where to get one for diesel engines...and would they cost a lot more? I know they are bound to cost more, since the pressures are very much higher. But I am thinking that buying one might be about the same as the cost of one compression check at a diesel speciality shop.