Well I have done a fair amount of research on lionfish...here is an article for you to read on choices The Lionfish Info Sheet: Captive Care and Home Husbandry by Frank Marini, Ph.D. - Reefkeeping.com
look through it post if you find one that fits and you like...I will also look for my old 55 gallon thread and post it so you can get an idea for cost, problems, and stocking(Without lionfish).
Also right off I want to say Lionfish(along with most animals in the hobby) require alot of care,time, and attention. So if you don't have that plus alot of money time to do MORE research and put your mind on on a few things that you probably are overlooking right now.
Lighting, Filtration, pumps, Live rock, base rock, sand bed, salinity/water conditions, top off, cycling, different algaes, and clean up crews(This is all right off the top of my head and should prob be researched before jumping into it) if you do this it will save you time and money...i wish I had... key things are time and money here IMHO
Feel free to post here or PM with questions