Hi
I agree with Mike, a Morgan any spec will hold its valve providing it is looked after correctly. I spent 30 years in the Motor Trade working for two major Japanese manufacturers and am fully aware of deprecation on motor cars. The first thing all cars loose is the Vat of the purchase price on driving off the forecourt, but there after it depends on the model and spec.
Buy a good spec'd and good colour Morgan and it will hold its value given time.
I have a Plus 8 bought in 2003 new and it is now worth 5k more than when I bought it but it is loved.
Good luck and buy the right car.