I would never recommend jumping straight into a KZ class with out experience in karting or tons of practice days before hand. As others have mentioned maybe a tag class would be better suited until you get the dynamics of karting down.
that being said…. prepare to hurt, and be tired lot especially on a sprint track. As Ayton Senna said its the purest form of motorsport.
If i was buying a shifter now, I would get something no more than 3 years old. CRG, Intrepid, DR, GP. CR125 is probably your best bet if you want to race. When buying used pay attention to joints, and cracks splits, anything that looks like its not an original weld. If you can have a well rounded kart guy come with to check it out.