Welcome to RCU.
How fast you ask? That depends on what gearing you have. Lots of
speed calculators to plug in numbers and give you an idea of possible speeds. If you are gonna be doing just speed runs, you could possibly reach high 40's with the right gearing. But doing so will be asking a lot from the motor and battery, and you will probably encounter lots of heat. For your situation, or anyone who wants to get the most from their setup, an infrared thermometer will be one of your best friends.
As for what kind of deal you got yourself, that depends on condition of RC and components. When I look at something used, I automatically tell myself the most I will pay for something is half what it would cost new.