If all you want is cadence* then any old cheap computer that has a cadence function will do it (Sigma, Cateye etc, I found the sigma ones better TBH but it’s probably personal preference).
BUT……. a GPS computer is so much more useful as they allow uploading to strava etc. Particularly the dual bluetooth/ANT+ ones. If you get a garmin with a cadence, speed and HR sensors, you can then use virtual power on apps like sufferfest, trainerroad, zwift etc.
OR, get bluetooth sensors and use those apps on your phone.
*distance covered on a turbo is meaningless as it takes no account of weight or aerodynamics. i.e. a heavy person will ride further on a turbo than a racing snake if they’re of a similar speed on a real road ride.