I think it goes without saying to look further south than your location if you want to avoid rust
For a car you can tinker with but can still be driven everyday you should expect to pay $5000 and up. At least that's what I found when I was searching for mine In Ca. I found one for $4500 that was a little neglected but well maintained and needed a complete dash out climate control rebuild which I did myself.
83-84 are the best of the best and most 85s are excellent as well but you have to remove some of the egr control and altitude adjusting doohickes to get the engine running at full power.
The big ticket problems with the wagons are;
1. The self leveling suspension the self leveling shocks in the rear are about $300 a piece so if you see an oil soaked rear shock expect to spend some real money. Inspect the pump itself located on the drivers side front of the engine for leaks etc. These pumps are unique to the wagons and
2. Bad cases of Hippie ownership where save the earth mentality drives them to insanity and they install WVO (waste veggie oil) systems and ruin the engines. I know a mechanic who is 35 years into a Mercedes diesel guru training program who puts new engines in the 300 series Mercs all the time for the same reason over and over. Blown rings and valve guides due to waste oil systems all because of the koolaide drinking hippie zombies who think they are doing a good thing. Start looking on craigslist at any of the 70s - 80s mercs and you will start seeing ads that say thins like "WAS running great on waste veggie oil, needs engine"
3. Vacuum actuated door locks and climate control systems are expensive to fix and are the most common thing to find not working on these cars. The vacuum "pods" are really expensive $40 - $130 each and there are something like 9 or so inside the car to operate door locks and air flaps etc.
Despite all the quirks these cars are built to be rebuilt over and over again and there is a reason there are so many left. If well maintained they will go to the 300,000 mark and still be running strong. Always change the oil and fuel filters every 3000 and be sure to convert to BIO diesel hoses and run a tank of Bio diesel through every 4th fill up or so cause it will help keep the injectors and pre combustion chambers clean. If a rebuild is needed parts are available but getting more expensive to buy genuine Mercedes which is starting to be "MADE IN CHINA"
My wife drives ours everyday and has grown to love it-
Buy the nicest low mile example you can and enjoy the ride