Hi Richard,
The Pre-89 rules are a bit more expensive to have a car Complied to in Australia compared to the personal import rule. With the Personal Import rule, the car just needs to have seatbelts with manufacturers tags, headrests at the right height (750mm above the seat base) and Child restraint anchorages fitted for any 2nd row seats.
With the Pre-89, you need to make the car comply with the Australian Design Rules of the year it was manufactured. This means Anti-Intrusion bars to be fitted to the doors ($350 to $600 per door depending on how hard the job is), new rubber brake tubes if you can't see the markings on the old ones, possibly a kph speedo if the original only shows mph and emissions system brought up to standard, which might mean new cat converters fitted, along with the Child Restraint Anchorages.
I am currently doing the engineering report for a friend who has brought in a pre-89 Bentley Turbo to Australia, so we have just gone though his modification list. The whole cost will depend on which Engineer you get to sign off the car in the state that you want to register it.
Unless the car is for yourself, I'd be surprised if you would make money on an XJS in Australia.
Also, if anyone is bringing a PI, try and get an Irish car to bring over, it has a kph speedo and so Australians would be more likely not to notice it as an English import. Australians tend to be quite snobbish about English imports as they expect them to be rust buckets due to the gritting of the roads and so you won't get the same prices as an Australian delivered car.
Regards, Blake A.