If the W210 has been maintained well, they should see 250,000 miles plus easily.
My tech had one with over 400,000 Kms on it. Mine currently has 156,000 Km and going strong. My MIL's 1997 E320 has over 200,000 Km (M104 engine, but the newer 722.6 tranny).
The M112 and M113 engines have been dissed as being 'cheaper' and 'weaker' as compared to the venerable old M103/104/119 engines but the reality is that they are excellent engines. Better or equal mileage with more power, easier maintenance and IF they have received proper oil changes, should last a long time indeed.
Mileage is similar, with M104 rated at approx. 20/26 and it's essentially the same for the M112. The M113 is rated at 19/26.
Larry Bible made an excellent post about the
M112/M113 engines here.
Although the 722.6 transmissions were dubbed 'sealed for life', many opt to change the fluid/filter every 50,000 Km or so. I have done so and will continue to do so. I would definitely consider the completion of this service to be a plus if I was looking at a used W210.
Good luck in your search!