Actually the Open 6.5 Class is a "low-limits" class, the limits are on overall size and material and more. The designer works within a "box" rule, for more detailed rules see the design section. Those boats are called proto, or prototype.
And then there are some plans adapted for home-building and easy of construction. Look for the Lucas CPmini, Rolland and Didi Mini etc. See the Proto Buying page for them. Normally very easy construction means slower.
There is a healthy amount of older designs sailing around. In the 1999 race year the oldest Mini racing in the circuit was out of '87. In 2001 a 1985 design competed. Andthat one showed up in 2003 again, and it was not even the oldest design.
What makes this class so special, i will focus on the boats, the community is awfully good, but hard to describe :).
Now you have a little feeling for what the boats look like, and do not forget the huge sails on a mast almost twice the length of the boat. Do you want to own such a boat see the page Proto Buying for several ways how to obtain a mini.