Welcome!
Some tips and pointers:
Definitely get along to an event, get a drive of a cyclekart and have a good poke around to get a feel for them.
Choose a pre-war inspiration race car
Have a read of the articles in the Start Here section of the forum
Have a read of Adrian's Builders' Guide in the Tech forum
In terms of plans there's lots of videos on youTube documenting people's build processes as well as build journals here and on the other websites particularly
http://www.cycklekartclub.com and
http://www.cyclekart-fr.comStart a build journal on here to show people your progress and to ask for advice about specific bits of your build.
If anyone does have a cyclekart for sale and that's what you fancy doing, then this is the place you'll it.