They are every bit as awesome as everyone says and then some.
If you're a tightwad (like me last season) you can run for $300ish per race weekend.
$50 to cover a $100 top-end every few races and a $300-700 bottom-end every season.
$50 for Saturday's entry fee
$50 for Sunday's entry fee
$50 average for broken parts due to fatigue over the season
$20 for aviation gas and oil at 100 racing miles per weekend and 40 MPG, water, driver food
$40 for a night in a motel
$60 for towing (160 miles round-trip)
If you intend to actually win sometimes, it's gonna cost $500-750 per race weekend.
$320 for the cheap race weekend
$200 for a set of tires
$80 more to stay two nights in a decent motel
$50 for a buddy's pit passes and food
$100 to the sky for extra crash damage per weekend running hard.
The definitive book on karting is "Secrets of Speed" by Memo Gidley.
The only karting website that matters is http://www.ekartingnews.com.
Don't get a TaG. They are grenades. The starter costs $200, the clutch costs $200-400 if it doesn't wipe out your crankshaft on its way out, the internals aren't as long-lasting as you'd think (one season on a bottom end isn't that far off).
There's no bad shifter engine. Most competitive karting uses a lightly modified 1999-02 Honda CR125 engine called a "Stock Moto". Fun karts nowadays use other 80, 125, 250 dirtbike engines or an "ICC"/"KZ2" which is an Italian 125cc engine originally designed for karting.
If you're moving up from racing cars to karts, you'll have an absolute blast on a kart with a Briggs LO206. Once you're winning on those, you'll know enough about the rest to make an intelligent choice about the next engine.
My opinion on chassis is that the chassis on the newest tires seems to handle best, and the chassis that is bent or cracked seems to handle worst. Don't spend as much on your trailer as you do your kart! It'll fit on a 4'x6' open-deck HF trailer just fine.
Physical fitness is paramount in a shifter. If you are not up to playing an entire Ultimate Frisbee tournament on a defensive line for a competitive team, you are not physically ready for a shifter. You could suffer serious injury without even crashing. Until the GoPro came out they would break virtually any camera you tried to mount - there is very little "vintage" shifter kart onboard footage. They pull well over 2g both cornering and braking, land off curbs at around 5g, and accelerate ferociously (0.8 seconds per shift in a famous Road and Track test). Get a seat that fits and a ribvest that works with your ribs.