There are others on this board who know more than I as to what to look for in a 560, but I will say it is a very good car and engine. More sophisticated than some other r107s ie ABS, ASR, air Bag, etc so there is potentially more to go wrong but there also will be fewer years of life on the car and it was certainely the pinnacle of the development of this model. I have a 1973 450SL, the opposite end of the spectrum.

Generally I'd say later years are best ('88 or '89), look for a clean body and interior and maintenance records. I'd say these are far more important than miles, but personally I'd take a peak at anything under 150,000 miles. Find one you like (with records) then get a PPI.

Nothing wrong with the 280SL Euro (if you can find one), I have a 280SE Euro and the engine pulls just as much as the 450SL (better than an '80 would). 380s are fine too but be certain it has the double row timing chain (OE on '84s and '85s, aftermarket upgrade on earlier ones).

Good luck.