Hey,
I raced karts, and still do occasionally and am at most meetings helping out, as my brother and my girflriend race. Best bang for buck class would be rotax lights or heavys (depending on your weight). You cant do much to the motors at all, just changing your jetting and barrel height pretty much. They're relatively low maintenance and very reliable, and electric start
. And they're quick. A reasonable second hand chassis and motor would probably be anywhere between 3000 and 5000 depending how new you want your gear. You'll need a set of slicks and wets, a racesuit (which is different to car ones btw), some sprockets, jets, tools, a kartsport license, and practise. Most things you will pick up as you go along.
Tires are controlled and are about $240 a set, but will last you quite a while. Karts are great fun and closest thing you'll get to a single seater and alot cheaper. Checkout
http://www.kartsport.org.nz for more info. Two tracks in Auckland, Rosebank domain and Mt. Wellington. Check the calendars and come down and watch sometime.
If you want to visit a shop to have a look at how much thigs cost, my dad runs the Auckland branch of Lascom Motorsport.
http://www.lascommotorsport.co.nz He can give you a good rundown on how everything works.
And oh yea, a few people I know do various forms of circuit racing, and karting is by far cheaper. Even the e30's are expensive, entry fees are $$$