just got my 1980 300d started
 

just got it started after sitting for a few years. it has 125,000 miles on it and i want to know what is the best type of oil i should use. should i also use purge to clear out the system? will that reduce the smoke and improve alot of power?

Congrates on getting the MB started up again.
How much fuel is in the tank? If there isn't much I wouldn't worry about it too much, just make sure you have a few extra fuel filters on hand. If the tank is full you may want to drain it and put new diesel in. I would adjust the valve, run a couple of cans of Diesel Purge through the engine. You might try pulling the tank stainers and cleaning it as well. It may be all clogged up with sediment.
As far as oil goes, it has been discusses over and over again here. Try some searching. What you'll probably find is that 100 people will tell you 100 different things. Everyone has their preference. I figure as long as you use a good name brand oil and change it regularly all will be well.

how many different types of purge are there?
 

is there 2 types of purge? one to use through the fuel lines and another to add in the engine?

Run Diesel Purge throught the injectors and IP only, pretty much useless to dump into the tank.
After sitting so long you are going to have algae growth in the tank and that can be pain. Do as Biodiesel300TD indicated and remove the fuel strainer, it screws into the bottom of the fuel tank, and rinse out the tank and clean that strainer. Use some algaecide in the fuel after that, you can find it at most parts store. I would use it in a few tanks, algae is hard to get rid of and the inline filters will show it. It turns that filters black. I would rinse the fuel lines out with some fresh diesel or some clean solvent or mineral spirits. Best to get that done before running the engine much. Then replace both fuel filters and the oil.

You might dump some fugicide (Biobor or Biocide, amongst others) into the tank before pulling the tank strainer. To kill the fungus. Then see if you get black sludge in the plastic prefilter. Then you will know if you have a fungus problem.
Algea won't grow in your tank, algae needs light to grow.

Products are made that you can dump in the tank to clean the fuel system, but to clean the injectors and the IP you are better off running Diesel Purge straight through the engine. DieselGiant.com has a DIY page on doing this. It's really easy.

I am a little curious as to why it was hard to start initially.:)