I just checked out what appears to be a great deal. It's an '82 Diesel
Vanagon w/ 208k miles, no rust. Engine looks relatively clean, no oil
leaks, good tires and breaks, tight steering, good electrical. Asking
price is $1450. The guy who ones it said he'd done lots of work on it (new
rings, etc.) and kept it in shape, took it on guys a few long road trips,
etc.

The the things that make me hesitant are: it's loud from the outside at
idle ( a lot louder that I recall my old '80 Rabbit diesel being) which
might indicate a problem (?); it blows a cloud of black smoke upon
heavy/fast acceleration  (which is clearly visible from the rear view
mirror); it's a slug -very slow acceleration and top speed of what
appeared to be 60mph.

I know that all that black smoke might just be an indication that the fuel
mixture needs adjustment, but combined with the loud knocking idle and the
sluggishness, it made me wonder about the compression. I also know that
diesel Vanagons are notorious sluggish. I don't mind doing a little work
on this car to bring it up to top shape, but I'm wondering if these
problems might indicate a more serious rebuild ($$$$). Also, when I
checked the oil level, it appeared to be well below (~1/2-3/4 quart below)
the low lovel on the dip-stick, which made me also wonder how the current
owner has been treating it (poorly?).

Should I get the compression checked?

Should I buy it and sell my trusty little '80 Rabbit that I've tinkered
with over the years and gotten to know oh so well?

Thanks for your opinion.