Welcome to the forum!!! A lot of this depends on what you are looking to do and how much risk you are willing to take. A kart that old there is always the risk of not being able to buy repair parts. For example, if something goes wrong with the braking system and/or when it comes time to rebuild the master cylinder or the calipers, the parts may simply not be available. I saw this first hand at my local track in the Houston area and the gentleman eventually sold his “great deal” and bought a kart and motor package that is ran/supported locally.
In regards to the motor, most shifters run a Honda CR125 (AKA stock moto) so support for the KX125 is going to be hard to find.
If you were just looking to go turn some hot laps than this kart package might work for you but understand that if/when something goes wrong you may get frustrated due to lack of support/parts.
If you want some help finding a good kart in your area than please let us know where you are located and if you are just looking to turn some hot laps or plan to race at some point. This forum and sport is full of so many great people that are willing to help out so just ask away!