To me paperwork only matters if the car is original and pristine. Having an "original" car that needs a complete restoration? To me the paperwork is nice to have, but a full restoration brings about its own papertrail.
Don't get caught up in the "investment" hype of an early car if its costs you $10K for the "car" and another 20K for the restore (and that number may not be accurate either). "T"s are the bottom of the early car interest (well, maybe 912s'
) The 73.5T targa is a bit more rare but right now the $$ is chasing S and E Coupes first.
If you are now going to "keep it forever" then don't get caught up on a number. Consider similar cars and what they are going for.. and how much work they need, to become the car you want this one to be. It could be after a 3.2, ST flares etc are added you might say.. "Boy, this Targa sure flexes alot. Wish I had a coupe !"
Sharing a pic or two would help the peanut gallery help you realize a market range price for the car as it is.