make sure you have the complete maintenance records. for milage that high i wouldnt pay more than $4k-$5K and only if i had records. for lower milage and excellent condition, i'd pay up to about $8K.
make sure the headgasket has been done, and the valve guides while they were there.
also, later models 94-95 (and some late model 93s) had wiring harnesses that were designed to be eco friendly and disintegrate when they got junked, except the heat in the engine bay started the process early and that results in all sorts of electrical gremlins.
get an independent mercedes mechanic to look at if first. a PPI will only cost about $100. well worth it for a car that can be a potential moneypit. in that link in the post above for mercedesshop.com look for a thread called "good shops/ mechanics in your area." call around the ones near you and see how much they will charge for a prepurchase inspection. also, that mechanic might have a line on another one for sale whose history they are familiar with.
some AC units can be faulty.
other than that these cars are stone reliable. however, if something should break you are going to pay out the arse to get the parts.
i love my 300E.