I have had a SFE in a 55 gallon before. You have to decide on a few things.
-If you want to keep the eel, AND other fish for a little while get a bigger tank. With A SFE, you need bigger fish, therefore a bigger tank.
-If all you're going to have is a SFE, and plan on keeping it long get a bigger tank...these things grow fast, especially if kept by themselves and fed correctly.
All in all, IME I would not recommend this for your setup, but then again, what do I know? You're probably going to do what you want, and pay the price for it down the road....good luck