For your budget, unless you're lucky, you're gonna be looking at high-mileage, not-all-that-well looked-after, probably cosmetically tatty examples. Obviously, the only thing really different with a diesel is the engine- I bought a diesel for longevity, economy and simplicity of the engine- the rest of my 124 is just the same as the 300E I had except it hasn't got a zillion problematic bits on the engine to contend with. They are also pretty basic to drive compared to modern cars, so don't expect performance anything like your 560, 'relaxed' would be my description...
The older diesel engines are so simple, a competent amateur can do the basic tasks like oil & filter/fuel filter changes easily. Well maintained, they do go on for ever, but the rest of the car will fall apart eventually just the same as the gas models
The usual caveats apply: like the others said do your research and find the model(s) you want, and the problems to look out for with that chassis/engine, go for the best example you can afford, don't buy the first car you see (unless it's amazing!), a high-mileage well-maintained car is better than low miles with no history and never driven- repair costs can mount up for these cars shockingly quick if they've not been looked-after... good luck