Reason R500 is discontinued (USA only) is lack of demand.
A PPI (pre-purchase inspection) by a dealer should be mandatory.
Maintenance/repair costs will be above average. This "goes with the territory." Any MB is closer to a Ferrari than it is to a Toyota. The not-appliance-like owership experience is worth the cost IMHO. The counterpoint to the notion that regular maintenance is expected of Mercedes owners is that you can find folks who can do it really well.
Ask your insurance man about insurance.
These cars have proven not really big sellers for MB (only 313 in November) and used ones are often bargains on the used car market. Especially those considered extravagant or gas guzzlers in today's mindset. This means the V8 and you should be able to get a lot of car for the money in this respect.
A lot of interesting "psychology" around the R-class. Americans I think perceive it as a "minivan" and there's little in this mindset that expects a "performance" minivan, which the R500 is. There's little also in the usual minivan mindset that expects anything but an appliance, too--that is, something that never requires maintenance. The R-class instead is a typical rather high performance car as is any Mercedes and with simply a unique shape and the possibility of three rows of seats rather than a trunk.
If you're thinking long-term, here's one little fact. The V8 has 16 sparkplugs and sparkplug wires are $35 each (per wire). Both items, however, are good for 100K miles.
If you're thinking DIY maintenance, know that no car is better supported for this than Mercedes. See http://www.startekinfo.com
where you can obtain access to all service materials available to dealerships as well as free access to the entire USA spare parts information system.