123/300D Performance guidelines & exh question

I'm contemplating the purchase of a rust-free 175,000 mile 300D. (Turbodiesel, automatic) For a variety of reasons, it isn't practical to get the car to a shop for a compression test. I seem to recall an anecdotal test i.e. the car should accelerate from 30-50 mph within a x seconds. Does anyone have any experience or specs for this?

Question #2 After test-driving the car, I opened the hood with the engine running and found that there was some exhaust apparently coming out somwhere under the aircleaner assembly. Is this be indicative of a known problem? The exhaust system is quiet, leak free and amazingly smoke free (for a diesel.) Normally, I'd remove the air cleaner and have a look around, but the elderly owner is very protective of her old car etc. Any insights would be appreciated. Thanks.
John J.