Heavy power steering on idle
 

Just bought a 2010 XC70 D5 and it feels like there is no power assist to the steering at idle. It seems fine when moving & there is no audible whine or unusual noises. The fluid is at the correct level although it looks black rather than green. It’s not a massive issue because it’s a manual so you can give it a few revs but I’m wondering if this is indicating an issue that’s going to cost me in the future?

Quite common... it can range from the gauze filter in the reservoir being blocked to a worn pump to the rubber pipes degrading inside and clogging things up. You could try cleaning the reservoir (the filter isn't replaceable) and flush the system a couple of times as that's the cheaper option.