Magnaflow,
Everyone has their own thoughts and ideas on what you will need. Compare responses and do your homework. Check out the articles page:
http://www.aquariumadvice.com/showfaq.php?fldAuto=2
Assuming you are starting out from scratch with just the 15gal tank I would highly suggest scrapping the idea of using the existing tank and check out a nano cube:
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=10677&N=2004+2020
These usually sell for $99-119 and basically come with everything you need minus decor and livestock. By the time you get the proper lighting and filtration for 15gal you will spend more than this. Considering a new 15gal is like $22 you are not losing much by not using it as most of your existing
FW equipment cannot be used with
SW. You could still use the 15gal for mixing/storing salt water and/or a
QT tank.
The new models of nano cubes with the fan and lights are great. I have been very tempted on more than one occassion when they have been on sale at my
LFS. They also make a 24gal model.
If you are going to set up a nano do not look at it as starting small and then moving up to a larger tank as a nano is an entirely different kind of set up with different livestock.
IME, when you start with saltwater you want to take your time and do it right or it just leads to frustration. Been there done that.
The Nano cube is a great affordable starting point to learn with as you prepare to do a larger display tank.
HTH,