Nice. A 125 gallon tank will be an excellent marine system. As you realize in your initial post, you will need to use a sump system on a tank of this size. I would suggest you look to purchase a tank with pre drilled overflows.
This is a big project, so rather than just talk in the general sense, how about you tell us what you are already considering so that we can provide some feedback.
By the way, on my 180 FOWLR I am using an ASM G3 skimmer. I have been very happy with the performance. I believe I ordered the skimmer from ASM G-3 Protein Skimmer* - AquaCave
Live rock will be another big expense for you. Keep in mind that you can use a large amount of dry rock, and then "seed" the tank with live rock. I used about 80% dry rock and 20% live rock on my 180. I order my dry rock from Marco Rocks The finest aquarium rock available, base rock, live rock, reef rock, marco rock, reef tank saltwater fish, live corals, Marco rocks, Fiji live rock, Tonga Live rock
. I used Key Largo rock in my 180.