I believe all 1982 911s had 85 MPH speedometers.
Looks like your's does not. If the speedo is not original mileage is simply unknown.
These cars are worth having. You should drive one and see. The SC is quick and a lot of fun to drive. The cars are also very durable.
However, if you need cold A/C and a smooth shifting tranny you have to go newer. Also, SCs cannot be chipped because CIS is mechanical. But newer usually = more expensive. Especially if you're talking 993s.