You want to scour the classifieds section. Here is an example of something that you might look for. This is at the low end of the price scale and used for a KPV with a chassis and full set up on the higher end will be $3-4,000. Without knowing the condition of this chassis, it is a pretty good deal. I bought the exact chassis model a year ago with a leopard motor for $2400 (I got ripped off, but didn’t know better). Got interested in F100 and sold the leopard for the same price I got the KPV (used) for.
If you are looking to spend $2500-4000 range, contact stu, he gets karts and basically strips them, paints them and gets all parts in great working order and sells them for about 1/2 price of new (but your kart will look new). He will also take you out and teach you how to drive, and tune it to your liking. Think of if kind of like a certified preowned car, since he will back lot of the parts for a little while (opposed to buying a used one and it braking on the first race and you are left fixing it on your own).
Actually I think my brother has a spare roller (everything but motor) he might be looking to sell, send me a PM if you might be interested in that too. I think with a motor you could do it slightly less than that ad I posted.