You will be able to pick up the original NES version for a lot less than Super Castlevania or Bloodlines. I would say start with the original anyway. You get to see how the games evolve that way.
SOTN is great, I have it on the 360. But like I was saying, the protagonists from CV3 show up in SOTN. Without playing 3, you won't have the context to really appreciate it in SOTN.