I currently have a unique view of the 'real' speedster world through the eyes of my son... His mom & stepdad are immersed in restoration concerns, expenditures of big dollars...Trying to buy low and sell high, all the while keeping their noses high in the air.
I've had three open air 356s and each was unique, one was actually in really nice condition, until I rolled it. I never lost a lot of money on them, but in that era, they were drivers not specimens. I did have my twin-grille roadster under insured when I had my accident, that was my big loss.
Two plus years now in my little pretend Porsche) and I'm not sure I'm worried about resale, knowing I am getting the car nearer to what I want to drive... The maintenance is negligible, most the fix-em-up has been from driving too hard.
From what we've all seen as the replicar makers have evolved and the various members have shared their experiences, if you have the money to tailor an IM or a JPS to suit you, man go for it...it can easily include roll-em-up windows, ain't nothin' money can't fix, ask GB.
If you have the dough to acquire a Porsche 356A speedster or a true 59' convertible 'D' or one of the 356 roadsters now on eBay, then you're going to have to factor in the costs and allay the fears you expressed, proper storage (although most insurance carriers want your replica stored inside as well)upkeep and swings in a very subjective market, classic cars!
MM