Popular consensus here is they will all be collectible. Mileage has an effect on F/V for sure, in the shorter term. In the longer term, many say that color and mileage will not have the same effect on value as they do today. 20 years down the road, these cars will be bringing far more than they do today, if things keep going at the pace they're moving at today.
Problem in your case, is that once you drive it, you won't be able to keep mileage down...it's a car like no other and you'll want to drive this car as often as you can.
Bottom line, every (non-salvage) FGT is and will be a collectible. Many here will punish you for not driving it (not me)
Everyone has an opinion, but I think most would agree that once you drive one, you'll want to drive it all the time.
I have several cars in my stable, some with more power and are faster, but none deliver the level of enjoyment the FGT does.
Good luck in your search!