You can always get a really good deal on a used Mercedes-Benz diesel.
You can get an early 1980s 240D for around $2000 -- 25-27 mpg with a 4-speed (don't get the automatic unless you don't care at all about acceleration, but otherwise it's a fine transmission). Or splurge and get a 300D of the same vintage for $2500 - 3000 -- 23-24 mpg with a turbocharger and an automatic transmission.
(I don't know if you prefer the European fuel consumption figures. Just divide 225 by mpg to get L/100km.)
They're indestructable. You may have to duct-tape a few rips in the upholstery like any old car but the engine will run forever. We've got a 1978 240D that we bought new and it's outlasted five or six cars in every family we know. Replaced the clutch a couple of times, a master cylinder, maybe a water pump or something like that too. When my Toyota died a few years ago, (yeah, Toyotas are cast iron, but Mercedes are made out of Kryptonite) I spent $3,000 on a 1980 300SD - the huge body with all the power accessories. It had 150,000 miles on it then and 200,000 now. And it gets 22-23 mpg. Actually if you ever need to carry five people in comfort with a quarter ton of luggage, you might want a 300SD. ^_^
Fuel economy is important, but you have to consider the whole package. The cost of the car itself and the cost of upkeep. Diesel engines are not "cool" right now so all the students and poor people are snapping up the old Benzes and laughing their way to the bank while the slightly more affluent people buy old Japanese cars that cost more and need more repairs before they're half as dependable as a Mercedes.
Don't worry about pollution. Even though they didn't have emission standards for diesel engines back then, Mercedes was used as the standard because nobody else could figure out how to beat it, even twenty years later.
If you live someplace where it often drops very far below freezing, forget this advice. Diesel engines won't start unless you find one with a block heater, and you have a place to plug it in at night. That's not bad at all, but it will probably be hard to find and the seller will know it's worth something.
