18' is doable depending on what load you are supporting above. I would have a rep from the local lumber yard come out to advise you. LVL will be your easiest solution if you are doing it yourself. Just need someone to look at in to tell you what size and how many ply you need for you situation. Don't use someones guess off the net who hasn't seen the house. Don't forget you are creating new load points that need to be addressed at the floor level where your new jack studs are.