The Moskito - A Two Year Lesson in Humility. will make for a good read.
I built a single seater from scratch on a budget and it is possible. I used a 521cc ski doo engine, CVT and a swing arm. I liked the rage buggies and stadiums and that was my goal.
Not sure if I would pick a zr700 engine for my next build. Not knocking the 700, but it is old. I have one bored to a 790, decker pipes and so on in a sled. 700's are very very cheap though. My next buggie will have a sled engine with push button reverse.