If you don't know diesels find one in good condition to test dirve.
I followed Craigs list for 3 months just checking prices and getting a feel for what mileage and condition was going for what price. My budget was 5k give or take a little.
I had never driven a diesel before my first testdrive. This is something I would now do first before ever testdriving a car. Drive a diesel, know what one that is in good working order feels like to drive. I had no Idea what a good ride was, how much shaking or car rattle was good or bad. If you are going to look at MB's find a mechanic first and ask it they know of any for sale, that they have worked on.
The problem I ran across is that the cars on craigslist were being sold to the first person who got there, and that in most cases asking to take a car for a diagnostic first was not possible, because there were 5 people wanting that car, no matter what shape it is in. I called on 5 cars before I actually got to one I could testdrive.
If the car's 20 years old and immaculate find out why. Get the maintenance records!
I was so frustrated with the process by that point i just wanted a car, and I got taken by a guy who lied to me, ended up he was a broker from Texas who bought this car in an auction. The body was immaculate, he had the car steamed cleaned probly the morning he listed it so that all the leaks it had wouldnt show while he was trying to show it. I paid probly 2k too much and have put in 1100 in fixing it so oil and hydrolic fluid dont shoot everywhere. All four motormounts (what attaches engine to car frame) were sheared and the transmission was shaking the engine all over.
Another place to look is Yahoo auto, Auto trader, Dealer websites in your area. Some of the little car dealers that are on main drags in towns. These places give you the chance to take a car to get a diagnostic first not alot of competition, slower pace transaction.
MP$ not just MPG.
Another consideration is MPG vs fuel cost. My MB wagon gets about 32MPG on 99.9% biodiesel. I have been buying from Western States in San Jose. I get fuel once a month. I fill up my tank and fill carboys up to last me a month. Last time I was there I paid $44 for one months fuel. My partner drives the same amount as me in a petro toyota camery. She gets 25MPG and has to fill about 2 times a month and spends $60 on petro. She paid last month about $2.35gal and I paid $3.31gal. I still spend less even though i pay a higher price per gallon.
For comparison sake I was in a 2000 Jetta TDI wagon on a trip to San Diego. The driver had filled up in Berkeley at the oasis, came by here to pick me up in Mountain View, we drove to scotts Valley to pickup another person, then drove down hwy1 to cut over to 101. We were getting 53MPG on 100% biodiesel. We got all the way to San Diego county limits and the fuel light went on, I go "oh do we need to stop" she goes no I got 2 gallons left that's another 100 miles we'll make it. We made it all the way there on 1 tank I was floored. I am so wishing I knew this was possible when I was looking, I would have gotten one of these if I could have.
I have considered taking the loss on my MB and getting a TDI especially on those very cold nights and no heater. Im still thinking about it.