I had a long nose butterfly for almost 4 years before it died and I bought it as a baby. In 4 years it was about 5 to 5 inches long and never had a major problem. They are very hardy and interesting fish to have and I would say go for it. Since I still have my notes from my inventory book on it I'll give you some info.
Long Nose Butterlyfish (Forcipiger flavissimus) It ferrets food out from deep between the branches of coral heads and in rocky recesses. They like food containing crustacea meat, algae, and sponge. They also enjoy using their long beaks to ferret food meat of fresh mussel or clam from the market. To get it started offer it live foods such as brine and Mysis shrimp. It is not for a reef tho. Very sensitive to water quality and if it stops feeding chances are water quality is deteriorating.
That said mine eat live and frozen brine, frozen mysis shrimp, flakes, blackworms, and pieces of diced frozen formula 1 & 2 cubes. It also liked romaine lettuce as a treat.
I originally bought that fish to cycle (yes cycle) my new 135 when I moved into our new house 4 years ago this week and it lived a full and happy life until a couple months ago when I found it dead one morning with no problems detected I believe it just was its time to leave.