hi, easy to get the cluster out, a couple of screws under the trim strip at the bottom of the cluster as you look at it, then some 10mm bolts i think (will go have a look, long time since i did it), then unplug all the connectors on the back. only took me about 10 minutes to get it out.
the odometer may be a mechanical glitch inside, not sure. i think the whole speedo unit only has a couple of electrical connections from the main board, so if the speedo is working it should all work i think. i didnt delve into the vdo speedo unit itself, though that would make interesting reading if you fancy giving it a go..!
i had fears of it all falling apart and me not being able to remember how it went back together..
i followed the smj autotechnik instructions and it all works now barring it needing a few new bulbs. turbo avant needs the same treatment now.
edit! yours is a 'square dash' model not 'curvy dash' but should be similar hopefully..?