Wow, jumping right back into it with a big setup. I agree you do not need a lot of things to get started with a fish only with live rock setup.
1. Look to get a tank with built in overflows. This will save you a lot of hassle with siphons and eliminate the chance of overflowing your tank in the event of a power outage. Built in overflows are definitely worth the money.
2. Buy a smaller tank for the sump. The biggest you have space for will help a lot. 55 gallon tanks work great and are pretty easy to come by used. Building your own is quite easy and much much cheaper than buying a prebuilt sump.
3. Buy the best protein skimmer you can afford. This piece of equipment is the most important of anything you will use on your tank. You get what you pay for with skimmers. Buy one that is rated a little higher than your tank size. I personally would suggest an Aqua C EV-240 with a Mag Drive 18 pump, but whatever you prefer in that range will work.
4. Have baffles cut out of glass or acrylic that you can silicone into place in the sump. You want to leave as much space for a refugium as possible while fitting the skimmer, pump, return pump, and heaters. The return pump will need to match the ability of the overflows to bring the water back up to the tank. Something like a Mag Drive 18 will work. You can find plans for Do It Yourself sumps if you search in google. The layout will depend on the placement of overflow and sump.
5. The refugium area of your sump will need between 4-6 inches of argonite sand in it. On top of that you will want chaetomorpha algae under a 6500 kelvin bulb that can be harvested. Both of these will help greatly in reducing nitrates and keeping high water quality.
6. In the main tank, an aragonite sand substrate will be put down. Depending on preferences and livestock intended, you can do a deep sand bed here too. Otherwise about a 1/2 inch should suffice. Nothing too coarse or extremely small though because it will blow all over or trap nutrients. On top of that, live rock will be added. Depending on what you afford, up to 200 lbs would be excellent. Aquascaping the rock to make caves, overhangs, and a scene you like will be made based on your preference and really help make the tank look nice. You can use pvc to support rock and build really cool caves and stuff. Again, searching online for this will be helpful. Base rock can be used on the bottom to save some money on the rock as well.
6. For lighting, metal halides are definitely not needed unless you are going to do a hard coral reef tank. Something like t5 or power compact lighting will be good enough for the rock if you want coralline growth. Otherwise you can use standard fluorescent lights.
7.A few powerheads to help flow and circulation. This will improve dissolved oxygen and keep waste floating in the air to be removed by the filters.
8. Proper maintenance will help greatly as well. Most important are 10% water changes every 2 weeks, then, cleaning filter pads and harvesting the chaeto as it grows.
That is all you need in my opinion. May seem like a lot of stuff but not nearly what some people suggest. You will spend much more money in the long run buying a cheaper setup and then upgrading constantly to maintain a healthy tank.
Price will vary depending on whether you buy the tanks used or new.
180 gallon tank with stand used 300-600 new 1000-1500
55 gallon used <100 new 150 or so
Skimmer with pump new 400
return pump new 150
sand about a dollar a pound
live rock retail 5-10 dollars a lb, 2nd hand cheaper
lights vary depending on quality from very cheap to very expensive
2-4 powerheads new 200-300 total used much less
Quarantine tank, heaters, baffles, etc. couple hundred bucks
Salt 50 dollars per 200 gallons
Most likely initial setup will run between 2000-3000 dollars, maybe more.
There are many other things you can add that will be helpful including RODI water filter, ozone, uv sterilizers, etc. But this setup should be plenty to maintain a healthy and beautiful tank. Lastly, I must say that a quarantine tank setup with a powerhead or sponge filter and a heater are very important/essential for all new specimens before going in a tank this big. You should quarantine for a month to make sure your fish is healthy and/or treat them if necessary. This is way better than having to tear your entire tank apart or worse, killing all your fish. GOOD LUCK!