The chassis and motor are both legal for the junior classes. (In fact if I were local I’d buy it and keep the chassis and sell the motor.)
For JB, you’ll need to run the small carb (Walbro WA55) which can be touchy, but can be set and forgotten once you get it in the right range. The large carb (WB3A) is much easier to play with and tune over a much wider range. All that said, the Briggs World Formula motor is getting favorable treatment for autocross and should be investigated if your daughter decides to continue into JA.
I can’t say anything about the pricing on it since you don’t mention how old the chassis is or how clean the package is. That would be a good price for a very clean and pretty new and unabused kart with the supporting items.