I think it also comes down to whether or not I really see this as a long term proposition.
If I think I am going to take this thing in, enjoy it, love it, and in 10 years from now still be as happy and nostalgic as what I am today, then it could be a worthy 'investment'.
If I were to buy a brand new cayman for 160k, how much could I get for it in 5 years? 70K? If I spend the same coin on this project, would I also only get 70k in 5 years? Probably. However I reckon in say, 20 years I could still get decent money for the mustang at that time. The Porsche would be a little worn and tired and close to the scrap heap by then (not to mention the money it cost in the meantime).
What am I going to get more enjoyment from?
Option 1
Buy new mustang now, 80k. In five years sell it for 30k. Lose 50K.
Then get the latest one at that time, 90k. Sell it five years later for 30K.
Then again buy the latest one for 100K. Sell it five years later for 30k.
In this plan, I would have lost about 180k on cars in around a 15 year period and own the latest of three generations along the way. I am sure the owning and enjoying would surely make me happy.
Option 2
Buy 'new' 67 mustang now, 180K. In 15 years sell it for what, 50k? Or drive it until I die? Do any of us really adhere to the "I promise it will be the last car I ever own"...???
In this plan, I would have lost about 130 on cars in the same period of time. BUT, I will have only owned the classic in all that time, not the latest three generations.
To help try and work out which way to go, perhaps I should also consider the phase of life I am in and what I am going to be able to afford, in time. Right now I earn good $. I can foresee that in a few years my income will be scaled back greatly. At that time, will I still have the money to spend on cars? Will I want to? Sure, I could take the argument that if I have 180 to spend now, just put 100 in the bank to wait for the next phase of life and the next purchase and still get a shiny new 2016 model right now for 80K which will happily see me through the next 5 years.
I don't know what to do, I really don't.
BUT, maybe that in itself is an answer?. If I REALLY was so totally passionate about something wouldn't I just say "I don't care what it costs, its what I WAN'T so that's what I'm going to have". Problem is, I can't say I REALLY want a 2016 GT either, but that's because they don't sell the V8 convertible in manual (and don't tell me it's not possible, because it's sold that way in the US).
Part of me also wants to be realistic about not being a greedy pig, and also getting value for money. However that sense of wanting value also only goes so far. For example if I think the perfect 67 convertible dream is going to cost 180, but I found some way of getting it for say, 100 to 120, I would probably jump right on it and never look back. But if I found out I could get a 2016 GT for say, 35K driveaway, I probably wouldn't even bother even though its half price.
Decisions, decisions...