Having never driven a boat before go for a mono. Monos are more stable, and are less finicky to weight adjustment and balancing.
I started with a catamaran, than bought a revolt 30. I sold the catamaran out of frustration, and kept my revolt30. I now own a miss geico v2, and a ul1 but my revolt is my daily go to boat.
As far as building a boat, I wouldn't do it as you're first boat. There's a level of experience you need to build a good reliable boat. Drilling holes in the boat for the hardware isn't something that can be guessed, and installing the stuffing tube is a form of educated art. Not to mention fiberglassing in motor mounts, servo mounts, battery trays etc.
I would, like rod, suggest a revolt 30 OR an impulse 31. Very good boats out of the box, and they have the ability to grow with you. For example, over time, I've taken my revolt from a stock 40mph boat, and gotten it well into the 60's (67mph fastest so far) with modding the hull (strengthening, truing, custom battery mount to lower battery in hull), modding the hardware (turn fins, rudder, trim tabs, strut), installing new (bigger) electronics and tuning the boat. The boat is just as stable at my modded speeds as a stock revolt, because of the mods I have done.