Have to admit that I kinda smiled at the idea of a dealer offering a guarantee on a 25 year old Morgan.... As to the worth of that guarantee... Who knows..? I guess much depends on the dealership and EXACTLY the type of guarantee provided, if one of those insurance based guarantees, best of luck with that..
As for a totally rust free 25 year old Morgan, I nearly spat my tea out...(-: My mid 80`s alloy bodied +8 was stripped and rebuilt having covered less than 8k miles from new, circa Y2K by a garage owner with Morgan expertise using stainless steel fixings and high gloss two pack black paint on the chassis bulkhead fuel tank radiator, and as you might imagine the rest of the paintwork was flawless when he finished rebuilding it, thus the whole car was built with more care and attention to detail than perhaps 99.9% or more of the Trad Morgans that ever left through the factory gates.... so much so it won a concours trophy at the Lakes Morgan Weekend circa 2003..
Currently with circa 35k miles on the clock and no surprise to me to find the occasional bubble beginning to show under the paintwork...
There can be a whole lot hiding under the skin.. I think Chris Harvey`s book Morgan the last survivor has some interesting content on issues that can crop up on older Morgans...
As for value, I suspect we may be into at least a couple of years if not more where baby boomers are trying to cash in their Trads, and with the passage of time that number would seem likely to go only one way, thus whatever bargain you might achieve, might be best to expect it to loose it`s cash value with the passage of time, even if you find a perfect unicorn...?
Other than the above, Morgans can create memories and friends galore. What`s more to want...?
Wishing you the best of luck in your search.
