My 560SL does 15-17 overall in mixed driving; though not everybody observes the requirement, MB says it must have premium. The 560 engine
AFAIK is as reliable as any of the others, and mine has been fine at 170K with only the expected/known issues--had the timing chain done once (an issue common to all the V8 engines used in the 107), had the radiator neck go and needed a new one, and needed a new starter. The top and bottom ends are fine and still factory with no redo having been needed; it'll burn a quart or so in about 2000 miles. Not sure how hard the fuel system is to work on, but on mine it's needed no attention whatever in the last 5 years and 25K miles.
Even the much-maligned 380 engine is actually as reliable as any other, from what I read, except for the single-row timing chain (before 1984) which needs either to be replaced more frequently or converted to dual-row. It's known for being less thirsty than any of the other V8 engines, and burns regular happily. Any of them will need regular attention to maintenance but will last essentially forever given that.
The last of the 560s is now a 19-year-old car, and stuff happens with any vehicle that old; if you're going to get into it, you need to be ready to deal with such things. Given that, I treasure mine--it's a great car to drive and I know of no two-seat drop-top that's more usable on a daily basis.
If it were for *my* Mom, I'd go for a 380SL with double-row timing chain, recently redone front and rear subframe mounts and engine mounts, no rust and great cosmetics, and the five-horizontal-button version of the ACC with a freshly resoldered circuit board (an easy 1-hour DIY even for me). What you should pay for that car gets you more fun and cachet for the buck than anything else I can think of.