Hi there, I actually race Superkarts (long circuits) and can safely say they offer unparalled excitement!
Firstly, I would very much recommend going to Darley Moor as suggested for that half day there. It is classed as a long circuit, but doesnt feature in the MSA British Superkart Championship, but is a great place to start long circuit karting.
Superkarts can be raced on short circuits which do indeed require a National B licence which is different to a car one I believe, but it is very easy to obtain. They dont have a massive turnout on short circuits however (especially 250s who struggle for numbers) but on long circuits there have been full grids of 48 karts (at Snetterton and Silverstone.) In order to race long circuits, you need a National A licence which requires 6 events on short circuits at 3 different tracks (6 events total, not 18).
As far as going straight into Superkarts, if you think you can do it, go for it! Perhaps get a cheap 125 outfit for your short circuit races, but 125s are not well supported on long circuits - if this is where you wanted to be. Darley Moor will allow you to see what a 125 is like on a long circuit.
In terms of budget involved, for a 250 (long circuits) you can pick up an outfit with all you need for about £3000 (a competitive one!) and I would say you can run competitively in a season for about £5000 a year. Short circuits are cheaper, but nowhere near as much fun in my opinion, for example you dont get to average 115MPH round Thruxton for example or take Copse flat out at 120MPH!
Overall, I love racing Superkarts, the atmosphere is very friendly, always someone to help (even closest competitors) or lend you parts etc. We always have a good laugh. But on track the racing is intense, but also very fair. 48 karts racing at once is quite a spectacle! Age ranges from me (18) to someone who is 63, so Superkarts are for everyone!
Here is an example video of me at Snetterton Qualy.
www.youtube.com/watch?v=KUXVLivRyPk
We will be (I wont be, I have the small matter of some A levels to do) at Donington on the 18/19th June, feel free to come along (I can probably get you a free ticket...)
Any questions, feel free to ask!