[jor]

No shortage of web info on the engine, which will either be the d24T or the Tic. The mileage really is nothing assuming use of synthetic oil, presumably the case or it would not have lasted this long.
Assuming general inspection is ok and the seller assures you that it's not losing oil, coolant etc it's really a question of driving it, which will put you in one of the 2 camps, lover or hater. A good engine will be (relatively) quiet, with a nice 'tinkle'. It's not to be judged on acceleration in the lower gears, but once it gets moving will be fast enough for most people. Overdrive just requires foot off the accelerator for a second. If it has lived its' life on the coast there may be some rust underneath and at this age you might need to be replacing shocks and springs - then again as it sounds as though the seller is an enthusiast this may have been done already. Same applies to gearbox and final drive oil. Cambelt is a bit of a job, but only needs doing every couple of years.
As regards parts, they are beginning to dry up in the UK, though a couple of breakers will show up every so often. Not sure how you would fare on the continent, perhaps ask Martin Calva who is in Falaise, France. Being a VW engine which has powered everything from horseboxes and campervans to gensets spares for the engine itself should be easily sourced.
If you want 'lots' of detailed engine information there is a US site devoted to the d24 and d24T (Volvo did not ship the Tic to the US), to be found at tufts.edu


jor