The future collectability answer is in the crystal ball.
No one can say for sure. I would say look at the steel bumper sharks & the circumstances it took to get their values going north as an indicator. All the C2's had to go thru the roof first. The rare C3's proly were always worth $ because of small production numbers. JMHO but a car is not an investment.
Maybe a good indicator might be the extreemly rare C3's like the LT-1. They seem to be holding strong and maybe on the edge of taking off. Maybe in enough time the ZR-1 will do the same but it's gonna take a long time for the ZR-1 if the C3's are any indicator. JMHO.
If the Z you buy was well cared for then the motor is the least of the worries in terms of it blowing up or wearing out. Just look at the number of 100K + mile cars still going strong. The trans and a host of other things will give you something to do way before the internals on the LT5 will break.....unless you buy a used race car ofcourse. The biggest thing you have to square with yourself is the seemingly difficult route to obtaining parts for the LT5 plus the ZR-1 in general and lastly the C4 as a generation. Also a C4 is more difficult to work on than a C3, in general. Once you get used to it that issue goes away but the fiirst little while you will wish for a less complicated car. The parts, ZR-1 specific, and C4 specific are out there as are the LT5 parts....they just cost more than C3 parts and are a bit harder to find until you get the hang of where to look. Look over at the NetRegistry site for some very important info about "common issues" with the Z. Go hang out on a C4 forum so you can see the general issues with the generation and add the two and then make your decision.
Oh, don't go driving any well maintained ZR-1's till you do your homework or you might forget about doing your homework!
This is my opinion on the subject and I came from the C3 ranks....I had a 72 LT-1 A/C t-top which I thought was all that till I got my ZR-1.
Good luck in your quest!
Tom