I have put an offer on a 2006 jag XJR with 41000.

First thing to do is check the service history, if it has had regular services by a qualified mechanic or even better by a dealer it should be in good condition, they are very reliable but maintenance can be expensive, keeping up the scheduled factory maintenance is important but this is where the expense creeps in, I would be checking your local dealer for their service prices, they will have a base price for each service, they can vary from shop to shop so best check where you expect to get the car worked on and serviced for their prices.