My advice is to start on a decent secondhand F18 like a Tiger. nacra F18n and older Capricorns will be fine too. Try to stay away from truly oldies like the Mystere Twister and Dart Hawk. I personally feel the Hawk is a very good basic design but it will need upgrades to make it competitive and selling a Hawk is rather difficult as most people think it is an uncompetitive boat.
Once you decide you like F18 class sailing then the time has come to consider a new boat.
Wouter