I'm your man. I've owned two of them. The 206d is a front engine front drive. They tend to have some front end problems. At least, the Frenchman I met who owned one told me he had to rebuild the front end periodically. The 207d's and up are front engined and rear wheel drive. These are a much more preferable version in my opinion. They had either the 616 engine or rarely, the 617. In both cases the block is the same as the auto versions but there are some differences. The IP is different, the water pumps are sometimes different, the valve cover is often different. They turn higher rpm's and have more horsepower. Only on very rare occasions were they ever factory turbocharged. The 207-409 versions are built on a similar small van chassis. The 50* and up are built on the larger van chassis. Sometimes the 50* had the 616 engine but they usually had the direct injection MB engines. Top speed on the 50* vans is usually around 57mph. The 207-409 will usually top out around 65-70mph.
I don't think you'll get any engine other than the 220d (615?) in the 206d versions. Transmissions are pretty reliable I believe. I had 5 spd's in both of mine. 1st gear is very very low in that tranny. Many had 4spds. They did sell some with automatics, but they are quite rare.
For a sense of what has been available, go to
www.mobile.de, click on Wohnmobile, then do a search on Mercedes. There are a gazillion of them availabe in Germany.
The 307d I owned in Europe, the 409d in the US. I recently sold the 409d. It was too small for 3 of us. In Europe they made fairly large class A motorhomes on these chassis. If you do the search on 'Hymer' instead of 'Mercedes' you'll see lots from the largest German mfg.