THEEE most important thing you should consider when buying a ZR-1 or any car really, is maintenance records and proof there of.... (ie receipts & or maintenance logs).
Dealers will usually not be able to provide these things, or very little information in regards to maintenance simply because it doesn't really matter to them, the idea is to make the sale & pay their bills. (this is not to say ALL dealers, there are specialty dealers that are more enthusiasts than salesmen, but only a few)
I purchased both of my ZR-1s from fellow forum members and both were enthusiasts. Both were out of state purchased and based on pictures and phone calls. I can say with that, I received exactly what was described. No surprises, no tricks or hidden bull crap....
Included were original parts removed, tools, memorabilia and stacks of receipts. In both cases for my 92 & my current 90, I feel like I actually received more than I paid for.
You see that I haven't spoken about miles yet. The LT-5 engine was hand assembled with some very close tolerances. Much care was put into the assembly of each engine by the folks at Mercruiser in Stillwater OK.
Major engine failures are few and far between... in fact, I really have not seen more than 1 and that was a member who had a head gasket failure on a heavily raced engine. I think it was Ron Kreig if memory serves.
The bottom line is miles on an LT-5 really dont matter especially when you can buy from a forum member and enthusiast. The enthusiast will be delighted to give you all the paper work and sit and talk with you about all that has been done to the car.
The bonus part of buying from an enthusiast is in addition to the maintenance records, you will probably have a car that has had some if not all of the normal ZR-1 issues addressed and some modifications to help with the smile factor when you drive it.
If I were in your shoes, the car I would most consider would be the blue one that FU has for sale. I am not friends with the guy and I only know him through the many years of being on this and the other forum. That said, I would give him a call and see what he has to say.
This is only one guys perspective and experiences.... Good Luck in your search and with whatever car you choose.