i run a mag 24 on my 115, and will be upgrading it to my ampmaster 3000 as soon as i get it back with new seals. a mag 24 on a 180 will be too small i think you will find, especially if you use it to power your skimmer.
on a side note, mags run externally are generally pretty loud when you get to the 18 and larger. mine is noisy, but i can't hear it over the other equipment when everything is turned on, so i dont mind. if you want to do it once, and do it right, get your tank drilled instead of going with an overflow box, and go with an ampmaster 3k. you will never be sorry and decide to redo it (a statement i can't make for an overflow box and a mag 12). the cost of getting your tank drilled will be less than buying an overflow box, and you can get it drilled anywhere you want, not being limited to having stuff run over the lip of the tank, interfering with your canopy/hood.
hopefully the tank is acrylic, when i did my plumbing it required 13 bulkheads to be cut, and it only took a 14.00 bit and about an hour of time, glass you gotta take the tank to a glass shop and pay them whatever they think a hole is worth, usually 20-30 bucks, but still less than the 75-100 for an overflow.