I may go to look at a W201 190D for sale that has been sitting "for a few years" (3 to 4). It's supposed to run and drive, but hasn't been started this whole time. I'm wondering if i should just drop a good battery in there and fire it up. Or are there some things i should consider first. Like the fuel in the tank for one. Does diesel go bad like gasoline when it's that old? Should i not turn the engine over with this old diesel fuel?
What other things should i be concerned about since the car has been idle for so long?

Diesel fuel can certainly go bad by growing algae.... even cars being used can do this... I think it would be worth syphoning out the old diesel and putting in fresh... but not much ... like three gallons plus a can of Diesel Purge.... and replace any filters you can reach easily.... Any filters you replace put Diesel Purge into them from another can.....This way you are doing a mini Diesel Purge injector cleaning right off the bat.
I would also change out the oil and filter if the rest of the car looks good enough that you think you want it.
Then if it starts and you buy it replace all belts and hoses as soon as you get it home. And flush the brake fluid completely before driving it. Is it auto or manual ... be sure to check the trans fluid level before driving it... if auto consider changing the fluid and filter and adding Trans-x.
What reason does the owner give for the car not being driven for that many years ?

Hello 300D-wanted-GA

In Georgia, you should be concerned with condensation = water in the fuel tank, and sludge growing in there, don't run off the tank..
A small amount of water will wreck the injection pump at startup.

#1. Install new battery.
#2. Disconnect the fuel filter feed line.
#3. Connect a 24 inch piece of bulk hose to the filter.
#4. Bring a bottle of diesel purge.
#5. Insert hose in bottle of diesel purge.
#6. STOP.
#7. Remove the air filter housing and have a board ready to seal intake, just a wise precaution.
#8. Attach 150 amp charger or jumper cables.
#9. Do NOT crank longer than fifteen seconds, with thirty second breaks between cranking.
#10. Watch the engine oil pressure during start, if there is no pressure at 5-10 seconds, run away from this purchase, unless you connect a mechanical gauge and confirm oil pressure.
#11. Start engine.

If you buy, add a new response to this thread, you will be flooded with advice.

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It's an automatic. It has some wreck damage to it. Owner said it was driven home after the wreck and parked.

Then you will need to check for frame damage/warpage at some point...
You should look for the factory specs and the places where you can drop a plumb bob from to a flat concrete floor.....and carefully measure them to see if you have REAL problems...