Welcome to AA! You'd definitely want to use a larger sump for your 120. The 15 gal
is just too small. You want a sump that will be able to give you an adequate amount of water flow for your skimmer to filter and hide your heaters in. Using a 30 gal
long tank would probably work better for you (when it comes to sumps the bigger the better) and if you can swing it maybe even a 55 gal
in case you want to add a refugium later on.
Your choice of skimmer is a good one, the octopus is highly effective so you'll be covered there. As far as the powerheads, check out the Koralia series. They can move some serious water and your looking to move 10x-30x your water volume per hour. Here's a link to some powerheads to help you shop a bit:
Fish & Aquarium Supplies: Powerheads
For the over flow box, if your not going to drill the tank, take a look at the eshopps overflows. They are great HOB
overflows that will feed your sump at a good pace.
Reef Aquarium Filtration: Eshopps Overflow Boxes
Lastly, you don't really need to upgrade the lighting if your doing a FOWLR
. If you plan to keep any corals or nems, then you'll need a lighting upgrade. Lights can get expensive quickly.