Well - 1st wait to see if you get the car.
One thing to note - if the speedo is broken, or at least is not functioning, the odometer will likely be off, and you will have no idea how many miles are really on the car. Just beware.
2nd - If you get the car you might want to see if the the problem lies in the speedometer itself or if it is just a cable problem.
3rd - If it is the speedo, I would recommend buying a used one from Ebay (either an 89 140 unit or an 87-89 85 mph unit) as a replacement.
4th - You can pick up the 85 mph units very cheap. Send it to Two Brothers Perfomance and have it recalibrated and refaced to a 170 or 200 mph unit for $70.00.
The Autometer unit, although a nice piece, is not just a bolt in unit. You will have to hack up your existing cluster and risk damaging the electronic units associated with it.
Hope this helps.