Mate, As ABU53 has said, check out some kart meets before you go and spend your dollars. There is a hell of a lot more to it than what you see on the surface. Your best bet is to get into a sealed engine class although I'm not too sure if there is such a thing anymore.
I used to race Rotax 125's and thought it was a great formula but like everything the cheats got involved and ruined the class. If you want to be competitive in Karting it all takes dollars and not a little skill as well. Most classes are governed by weight limits and it is really amazing how much can be gained by shedding (or adding) kilos.
Then you have kart setup which is a bloody science all to itself. Tyre pressure, axle size, axle stiffness, KPI, Camber/Castor, age of your axle bearings, position of your seat, roll bar stiffness. pods tight or loose Ahhhhhhhh used to drive me bloody crazy. Great fun if your content to be a midfielder but bloody frustrating if you want to run up front with the big boys.
Oh yes and don't forget you will need a trailer, quikerect sunshade, race day fees, club joining fees, AKA fees, all the mandatory driving gear and heaps of money for consumables like chains, sprockets, plugs, fuel, oil and tyres and on and on and on. You will probably need some sort of Data logger like an Alfano or similar to record all those splits etc. Oops speaking of splits, almost forgot about the engine rebuilds ummmmm costly buggers they are. And don't let anyone tell you that racing J's or Clubman is cheap either. If you really must do this then buy a good chassis like an Arrow AX-8 that will allow you to do a couple of classes.
So your 3-5 grand for the first year has just become 10 grand in no time at all.
I had a ball but I could have brought an R33 GTR with what I spent on karting.
Muz