At 4500 hrs, my TMD40s were in great condition. I had some issues with the rubber boot that is on the Heat Exchangers, but by then had solved that issue. I didn't have the aftercoolers on those engines, but have them presently on TAMD41s.
If I was looking now at a pair of TAMD40s, at 5500 hrs, I would want to know that the HEs were rodded recently and the boot was properly seated. On mine, that was the cause of some overheating until I diagnosed and fixed those issues.
Others on this forum, in various parts of the US, have complaints about support from Volvo. Here, I had excellent service from my local Volvo Penta mechanic, but really, parts and service are needed so rarely, if you do your own oil changes, filters and HE servicing, I don't see how that could be a serious issue. If you don't have a mechanic you can trust and do nothing yourself, I doubt there is an engine of that age that would make you happy.
I had no trouble selling those 40s when I moved up to the 41s.