I just designed and built my own. It's 80% complete but it's to the stage i have been launching it.
It's great, happy as I can be.

I was'nt sure what I was doing or what the result would be...a few hundred hours later and it's all good.

But, sounds like you are trying to get a specific result from your build that will be tough to achieve on a first try.

But then again what if the pre designed boat you build isn't what you expect...I'd try the mystery or similar
to see if that's what you like.

Honestly though, you sound like you want to design your own. DO IT. Use similar boats
as a model to keep you within normal parameters. Be prepared to accept it's quirks when done and know you
can change it's traits somewhat to adjust...ie move your cockpit if you have to, shave or add to the stems, etc.

and after you finish it, you will know what you could have done better..which means you will start on the next one.