It will be a lot more expensive to build a ski from scratch. It'd be cheaper to buy a complete ski and get it shipped. Parts for SJ's are readily available from just about everywhere since they are the most popular. I knew the SN had a top deck made from Metton and the RN was made from SMC. Neither are great materials but they both hold up. This is the first I've heard of the RN being lighter than the SN. They're both listed from the manufacturer as being the same weight. The RN hulls have more options for pole brackets, poles, and hoods. However, you can put a RN pole on a SN hull with something called a hazard bracket...and why anyone would want to spend $1000 on a lightweight hood for a stock hull is beyond me. The midshafts from 90-93 are different from 94+. The SN waterbox has smaller inlets and outlets.
IMO, buy the SN and save a couple grand. Then use that extra money for mods and improvements until you decide you want to get an aftermarket hull. $3000 will buy you a really nice heavily modified SN that will spank a virtually stock $3000 RN everyday of the week.