I bought my '91 300d 2.5 turbo with 99K miles and all records from an independent MB only shop. The seller was private, but the shop had worked on the car for 5+ years and had carried out a 50point inspection, and all records were with the car. All servicing had been done as scheduled.

I have put 40k miles on the car since I bought it. I have replaces belt shock, alternator and injectors. All to be expected with this many miles (well, not many for this engine). I have been very happy with the car's performance and manners; a pleasure to drive on the highway, economical, adequately powered and comfortable. I also have a '79 300SD with 360K miles (my purchase of the 300D 2.5 was the result of 16 years ownership of the SD); the '79 still runs great and seems to have a bit more oomph -even after 360K miles- but the 2.5 is quieter, smoother, more economical and well, just more civilized.

And yes, it is a turn it on and go car. As someone else said, these cars are hard to find, but since they share most of the same parts as the gas 300E's parts aren't a problem. Great car if you can afford to maintain a 'benz... But pass it by if you don't know that it hasn't been looked after (i.e. if the service records aren't with the car)
Good luck