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.
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The compression gauge must have a range up to 350 psi for the engine's spec. is 319 to 349 psi with 44 psi variance allowed between cylinders and 218 as the minimum allowable compression. Tester is connected to the glow plug opening.
It needs an overhaul when it burns oil and is hard to start despite a recent valve adjustment. That's when you need to act.
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You can get one at HarborFreight.com or check internet for better prices.
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