I have a 1984 300D turbo and I want to learn about modifications I can make to this car. I know it wont be cheap but what are some of the first steps to take to getting a little more performance out of this car? I don't plan on making a super expsensive hot rod but just a little more grunt. I know very little about diesels.

Assuming your injectors are in good shape(good pattern&balanced), your fuel tank screen and fuel filters are clean, and your engine is timed properly:
Free:Tune wastegate back to spec (If you have a Garrett install a boost controller) around 12psi
adjust the ALDA a SMALL amount for some more off-the-line performance(with a potential reduction in economy)
Make sure the transmission is getting the proper vacuum signals and that the bowden cable is adjusted properly. Shifting into 4th before you hit 40 means its getting the wrong signals.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Thank you sooo much for this info, I don't know about the injectors or the timing, all I know is it shifts just fine. I know very little about diesels and I have learned a lot so far. How would I know if its a garret turbo?

If your injectors are original then its more than likely time to rebuild them get a hold of C. Sean Watts or bgkast on the forums if you want someone else to do it for you.

Check Alda for leaks,and check turbo waste gate hose.Then check your boost.I had been driving 2 years on only 5 psi.But a 25 year old turbo needs adjustment.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

First and foremost:

Read and research

You will find nearly all of your questions have been asked before. After reading you can then ask specific questions on items and issues you fail to understand. No one can really formulate intelligent questions unless they have some information, knowledge and ideas to base those questions upon.