Bose-X, I agree that depreciation is another factor you have to take into account. And this is possibly an even more complex subject than the valuation of the engine fund.
For years, the depreciation on the 30-year old spamcans (PA-28s, C-172s) was virtually negligable. But since a few years I've got the feeling that they're depreciating like mad again, due to two factors:
a. The advent of modern composite airframes (Diamond and Cirrus being examples) and a whole class of new ultralights, microlights and whatnot that are far cheaper to run & maintain your license for.
b. The advent of new engines, particularly the Rotax 912 and Thielert diesel, which all promise to make fuel costs in the aircraft that have them, far less. And generally live up to these promises (admittedly with a few teething problems).
As a result of this, the average 30-year old spamcan now has so much competition from newer designs that are cheaper to operate, insure and sometimes even to purchase (2nd hand spamcan vs. new microlight) that they now depreciate like mad again.
To give you an idea, at my club the IFR DA-40 is cheaper per hour wet than a VFR PA-28! Now you can discuss the flying qualities and training properties of the DA-40 vs the PA-28 but my wallet is the overriding factor in choosing the DA-40 if it is available.
For me, at this point in time I would not buy a share in a 30 year old spamcan under any condition. If I had a share I would try to get rid of it while I could still find a sucker to buy it off me for a good price.
I've got the feeling that a lot of owners of 30 year old spamcans will be very disappointed in the years to come, when they're trying to sell their airplane/share and find that simply nobody is interested anymore.