Hi FZ, I think you may have misunderstood the point that JOE-DOT-COM was making with regards to your enquire about value of the car.
The point being made was that the car is not necessarily worth extra monies due to it being modified, in fact many serious buyers of Jaguars of this era will be looking for Original Equipment as it was built and left Jaguar production line. It is often originality, mileage, service history, as well as the all important condition that dictates what a buyer is prepared to pay for any given vehicle. The point being made about modified vehicles is very true and very, very important regards insurance. Quite often insurance companies in general will not want to quote for insurance cover on modified vehicles. The specialist insurers that will/do quote are very likely to charge premiums for the cover. Also consideration has to be made to the fact that if modifications are not declared, then the insurance would/could become null and void, in the event of any incident.
Best Wishes and Regards, John