Here is another 73 T Targa that will probably hold it's "value" better then the one you have your heart set on.
http://forums.pelicanparts.com/showthread.php?threadid=326878
I do not know if it has been sold and it is a bit more but at least it appears to be rust free and safer to drive then having a light car with a Turbo.
Were the brakes/suspension upgraded on the car you are interested in?
I gad my heart set on a n early Targa one time, culd not keep the thought of the car out of my mind. Took it for three test drives, placed it on a lift to check out the under carriage and was about 5 seconds from placing a deposit until someone spotted bondo in the kidney bowels and in the rear quarter panels near the tailights
I would reccomend reading the book by Zimmerman, The used 911 story.
Good luck.