Chassis should have a CIK homologation number that will give you a range. Be aware that owners often have poor memories of when they bought stuff, your best bet will be to find out results from the last time the kart was raced.
The KT100 is a good beginner motor, I started with one in 1977;-) But, unless it’s been freshened recently the value could be suspect. I sold one to Walt that came with the kart on my website, they really need to be taken care of so if you plan to race it and can’t talk to the last builder who worked on it you will be dollars ahead to have it freshened by a pro before running it. Also the clutches can be quite spendy so make sure you have something decent that will suit your weight before heading out on track.
If you’re close to the track have the seller meet you there on an open practice day so you can test it out. If it hasn’t been on track for a while and the seller isn’t interested in doing this the price should be a lot lower. Just driving it down their street won’t tell you much, going out on a practice day will help you find out if the kart is legal for the class you want to run and if the seller is asking a fair price. Talk to others in your potential class about the package for an understanding of what it will take to actually race in terms of tires, equipment, etc.
Remember, the cost of racing is much more than just buying the equipment. If you go into it knowing what rebuilds will cost, how often you need new tires, transportation costs, practice fees, race entry fees, and miscellaneous things like pizza and beer afterwards you’ll have a much better time of it. Post a couple of pictures of the kart if you can, also let us know where you plan on racing since there might be someone on here that knows the kart in question and can give you an honest assessment. We all want you to stay in the hobby and be successful with it and getting started right is critical to that happening…..