Hi PingPong and welcome aboard
As someone that built already 13 boats, 11 of them Fiberglass/Plywood construction I want to say a few things:
* I didn't find free plans for any "Real" boat of any kind except some dinghies, kayaks and canoes.
* You may try to find a design that you like that have enough data you can make something close to it. I am using a CAD program and DelfShip, DelftShip is giving you everything you need to make a good design, you have to learn how to work with.
Try a search in the link bellow:
sailboatdata.com/firstpage.asp
* Checkout the link bellow with Bruce Roberts designs.
I am not in any way connected tho him
www.bruce-roberts.com/POWER_CA..._PLANS.htm
* You may want to see what are the available catamarans on the market. In the range you are talking about they are very expensive (for me in any case). Take in consideration that the big money is not in the construction itself. The big money is in the details, from engines to furniture's.
* You may want to build a small catamaran with the techniques you would like to build the big one to get some experience in the subject.
Best wishes
Iosif