Is this expensive to repair? Should I buy a new stove/oven?

The F31 error is just a shorted temperture sensor. It's a cheap repair and not worth replacing the stove for. The sensor you need can be ordered from the link below.

https://prismparts.com/products/for-kenmore-range-oven-temperature-sensor-pp5349693?_pos=1&_sid=e0a4eabd6&_ss=r

The page linked below shows how to access the sensor.

https://www.appliancecodes.com/repairs/how-to-troubleshoot-and-fix-kenmore-range-f10-error/