[welcome]
Michael!
Since you've gotten your feet wet with a fish only, I think you probably should drill the 125 and set things up for an overflow with a sump. I really like that setup compared to a sumpless setup because the aquarium looks a bit cleaner, the water level stays the same in the display all the time, you can put addititives in the sump or into the overflow so they mix well before going into the display, etc. u-tube overflows are an accident waiting to happen that can work well, but a regular overflow is more worry free. I can't give you advice on how/where to drill, but others on here can.
RE skimmers, I like how the octopus skimmers work. I have a Tunze 9010 that works pretty well. Euroreefs are nice ones as well. As far as bang for the buck goes, the octopus skimmers seem pretty nice.
You'll want to think about lighting, temperature control, test kits, stability of the environment (temp, alk, ca, etc.), sand bed type (none, shallow, deep). DawgDiggity is doing a build right now and is dealing with a lot of the same things, so you might tag along with his build
