You have a great bargaining chip - the current owners most likely got a quote to fix the fan and this pushed them over the edge to get rid.
To due job Volvo way as said requires removing dash so could be upwards of a £1000 to fix at a dealer; an indie should probably manage it for around 1/2 that but you have a good case to knock a grand of the price and do it yourself, allowing you some quality time with your new car!!
Then when you have fixed that, you most likely will need to sort the hand brake which is the achilles heel on all MK1 XC90s as you likely know already
