I don't think you need to upgrade your lighting at all. You have enough lighting to handle most any coral.
The tang will be ok in the 55 though would be happier in a larger tank.
The biggest thing I would do is work on your filtration. Since you are going to go with a reef you really need to consider adding a sump and refugium to your system. Click on my thread and read through some of the things I did to my system. The wet/dry trickle filter just doesn't cut it for a reef tank.
How much LR
do you actually have? What are your water parameters? A reef is more demanding in that you have to keep your CA
, Mag and Alk
in check. You will need these test kits along with a refractometer if you don't have one already.