The values of used Morgans have never made sense to me, I had a similar struggle when I was looking for my Plus 4, the age, mileage and price never seemed to correlate in a linear fashion. In the end I found one a standard spec model that the price was reasonable on. The improvements came later!
On the AR upgrade, it's well worth doing, in my case, more power and better MPG, whats not to like! It makes the car be as it should have been leaving the factory, athough you may need to improve other area's to make the most of it etc,
As an aside, it makes it tricky to get the swapping bug for the new Plus Four, although a test drive may change my mind, however the price means I willl be staying put.