I agree with Chuckit!
Best way of sorting out buying a bike is to get along to the meetings, there are bike available to buy, but most riders are wary that the bike will end up in a shed.. or on a stand... or worse still, on the road! it would be a disaster to loose even one that way, so to show that you are keen is the best solution and you will be passed onto the right person.
We need new riders desperately, but do not want to loose the bikes.
See the main web site for the dates of the meetings.
If you decide to build, then there is advice to be had here on the forum, and plenty of articles on the main web, but you need to look at the bikes being raced now to get an up to date picture of where you need to go to build competitively. Betters still pal up with a couple of riders and builders...
Hope this is helpful.
Alan