Sounds like a previous owner did the conversion. A stock Euro would be a 4-speed, like "grey500se" just said. I bet it's a lot of fun to drive, though...better than a 4-speed, and WAY better than an automatic!!!
Aside from the transmission, to determine if the car is a Euro--it will have the smaller bumpers, and the oil and temp guages will be marked in "bar" (1-2-3), and "Celcius", instead of "psi" and "farenheit". Fuel guage will be different as well, and probably some other details that I'm not aware of. There's a few Euro owners that post here...maybe one of them will see this and can give you more details to look for.
As on any older car, good maintenance records and documents are a plus...check the climate controls...if they don't work, that can be a high-$$$ repair. Look for smoke...A little smoke when you first start it is normal for an older diesel, but it should go away as the engine warms up, and there should be very little noticeable smoke while driving, except maybe when you floor it pretty hard. Make sure to test-start it COLD, to make sure the glow plug system is working well...and that's also a good way to ensure that the compression is probably OK, and that the valves have been adjusted at least somewhat recently (MB recommends valve adjustments every 15,000 miles for thier diesels).
Also keep in mind the front suspension...If it has over 150,000 or so miles, and the front end hasn't been redone with new bushings/ball joints, etc., then you can count on having to get that done in the near-future, to get proper alignment, handling, and to prevent excessive tire wear. That also is an expensive job. How are the power windows/sunroof/central locking, etc...these are also common problems. Check for hidden rust, of course...look under the floormats for signs of leaks and/or rust. Look at the air filter, and the clear fuel pre-filter down by the driver's side of the engine...if these are dirty and very old looking, that can be a sign of poor maintenance, even if the engine itself has been cleaned and detailed by the seller, these little items can be incriminating. Obviously a lot of these things aren't strictly diesel-specific, but I'm on a roll here!
I would definitely have it inspected by a reputable shop, to make sure the conversion was done properly and everything is in working order, and not a hack job. If it's in good shape and is priced appropriately for its condition, I would say "Buy it, fast!" That would be a very unique MB to have...American-spec 300Ds never came with a manual shift of any kind, to the best of my knowledge.
!!!!! Can I drive it one time?....