Abnovak,
Since you are near the car and it did not sell on Ebay -- you need to seriously re-consider it.
What you are looking at is a 1966 Jaguar 3.8 litre S-Type made from 1963 to 1968. There were only 25,033 S-Types made of which 15,065 were the 3.8 litre. There were only 3685 made in 1966. These are magnificent cars and they will outshine and out perform the Mark II -- hands down. Because the Mark II is the popular model, the S-Type can be picked up for a lot less but it is starting to be discovered as a much better car than the Mark II and it is starting to rise in value. Some collector magazines are now actually starting to value the S-Type and 420 higher than the Mark II. You will not see that train of thought in Jaguar circles yet, however.
There is nothing too wrong with this Jag. I don't understand why you would make the arbitrary statement that it would need a new suspension all around. The MOST the car would need is new rubber and perhaps rotors and pads. In a worst case scenario, you are looking at wheel and differential bearings. I can not imagine you needing all that, however. My 1967 saloon with over 100,000 miles when I bought it still had the original everything (except pads and rotors) and I drove it 600 miles home after I bought it. I replaced all the rubber because I wanted to tighten everything up and use the car as a daily driver -- not because it had to be done.
The rear brakes, with the parking brake, is the exact same parts as used on the XJ6 until 1987 and the XJS until much later. Slight differences are in the configuration of the rotors and the pads. The parking brake parts are all the same. There is no reason the parking brake should not work unless it needs adjustment or there is a broken part.
The transmission is a Borg Warner Detroit Gears DG250 direct drive high gear. It was also used in a variety of US cars -- most notable the Studebaker line. If it is operating in all 3 gears and reverse but not Park than the cable has slipped slightly. There is a metal rod attached to the interior linkage so that when the selector is in Park the rod "locks" the transmission. The cable is MUCH like the ones you see on 10 speed bikes. If the hold down for the sheath is loose, the selector is slightly off and the inside metal cable does not extend completely. There is an inspection port on the side of the tunnel which, when removed, exposes this hold down. It is about a 30 second adjustment and the Park position will work again.
Parts for these cars are all readily available. They are as easy to work on as your MG. This particular S-Type appears to have the vinyl interior (Amblahyde) not the leather. While this is less desirable than leather -- it also wears like iron and will not need to be replaced like the leather usually is. You also won't have to constantly maintain it.
These are fantastic Jags. If it is running and you can put it on the road to drive it, it is well worth more than the price it got to on Ebay. What it is worth, however is between you and the guy selling it. It won't, however go down in value as time goes on.
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