im not very good with skimmers for that size tank.

as big as you can get IMO, i always feel like i need more space in my tank. more water volume add a lot of stability to the water chemistry. my dream is to have a huge sump so i can have a spot for equipment, a section for a frag rack, a section for a refugium, and then another section for equipment... maybe look into an external? im assuming your 180 is dual overflow, so you will want about 750 gph per overflow after head loss and what not.

first i wouldnt buy a korailia, they break way to easy and are expensive in the long run. many of us just switched to this new brand named Jebao. i switched about 7 months ago and love the new one. for my 90 gal i bought a WP-40. its something like 3200 GPH. for your 180 gal you could buy a WP60, it will be a lot cheaper than a few korailias and give you a lot more flow.

your plan sounds cool.

if you want to save money on lighting in the long run i would look into LEDs. obviously they are a bigger start up cost but they last for about 5 years, and ofter that you can still replace the diodes pretty easily. diodes are about $1.50 to about $3.00 depending on which kind you want. i personally am using the $1.50 type over the past year and i havent had a single one go. im using LEDs made by reefbreeders, i would reccomend checking them out, for a 180 you could do well with 3 16" fixtures

sand beds tend to be the opinion of the hobbyist, i have a shallow, some have deeps.

many people still run refugiums, if i had the space i would definitely have one.