I recently acquired a 250d with a bad odometer however a brand new
speedo came with the car. I'm having a hard time figuring
out if the new speedo is a correct replacement.

The old speedo is

A 124 542 17 06 (0-140mph)

The new one is

A 124 542 79 68 (0-120mph)

I have the epc subscription allowing access in international cars,
but there doesn't seem to be a way to find out which speedos work
with which rear ends (I assume that's the key criterion).

I suppose I can test it with a gps, but I'd still like to know how
to look such things up properly

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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000

I guess the speedometer will match the rear differential ratio. Perhaps a dealer can tell you more?

Otherwise look out for W124s with the 120mph speedo - duck down underneath scrape off some muck and read the number stamped on the back of the diff! Sorry to give such an impractical practical solution...

I have to say, epc is pretty hard to use for something like a speedometer.
Here's what I've discovered:

My new speedo A 124 542 79 68 correct for the 250D

The speedo I'm replacing (put in by an import conversion service in 1987)
is A 124 542 17 06 was originally used in the 230e

If correct, the part I'm replacing was intended for a 3.27 rear end
instead of the 3.42 rear end in my car. Which was about a 4.5% error

Given a model designation (e.g. 124.125 for my 250d) one can
search for the applicable parts by name (e.g. speedometer). Going the
other way can be tough -- using a part number to find the applicable models.

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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000