The Soviet/Russian R-7 (NATO SS-N-6/SERB) has been around since the early 1960's. It is a two stage, liquid fueled missile that has been considered obsolete for ages. Liquid fueled missiles do not provide a very good response time as they must be fueled before launch. Using so called "storable liquid fuel" missiles is very dangerous and can result in a major BOOM! Just ask the Air Force about what happened to the Titan II missile in Arkansas.
If they really wanted to build a decent missile based on a Soviet design, they'd go with either the R-36 (SS-9), the R-36M (SS-18), or the RU-100N (SS-19).