Don,
Lets talk real life here.
You are looking to down size from 300 (which in my opinion was way to big for the space you have) to a 150 or 180. Both 24" in deep and 24" high. Buy it new and save yourself problems like you had in the past with the 300. Buy a standard tank that will fit your room. IMO a 125 if you want a 6' tank Reef Ready or maybe a 120 Reef Ready which is 4' long both will be easier for you to work on and would better fit for your room. Buy either a AGA or Oceanic at least they come with a warranty. I know that AGA honors their warranty. I had a stand collapse and they replaced it. Luckily I was taking the tank down when it happened so no water damage)
For the stand you can build your own or buy one to your liking but make sure it is well built.
You have a pump that will do great for a closed loop. just make it simple (4 returns and one feed). If the fish are getting sucked up into the strainer then you need to rethink your setup with the type of fish, if any you want to keep. I am still thinking that you want to go BB. If not that pump you currently have or any of the larger pumps you are talking about would blow sand all over the place.
You have 400 watt MH lighting. with the 125 you will need 3 and then you will have to deal with the center brace. with the 120 you can run 2 of them and not worry about the brace because you can put one on each side of the tank. Notice I am talking about standard glass tanks not custom tanks.
You have enough rock for either of these two setups, so no need to buy or add any.
You have a very good skimmer so you don't need to buy that. It will be more then enough for either tank.
Buy a new (not used) chiller in about the 1/3 hp and you should be in good shape with the MH type lighting you have.
Now your sump. you don't need a super large sump if you are just going to use it for equipment. Take it out of the laundry room and place it either under the tank or next to the tank. The soap, bleach, etc.. is or could be causing you a most of your major problems. If it doesn't' fit under the tank either build one that does or buy a cheap one form glass cages or some place else.
I think you have a Cal reactor. It can be used on either tank and is more than enough.
Now if you go with the 125 you will not need 400 watt MH you can use 250 hqi or 250 MH or T5's. If you go with the T5's you should not have a heat issue. I have 5 bulbs over a 40 gallon and my tank stays about 2 degrees above room temp when all the lights are on. Mine is the hood type that has a cooling fan built into it and yes each blub has its own reflector.
What happens with a lot of people is they forget that this is a hobby not a full time job. So keep it simple and you will be a lot better off. I know I did it both ways and a lot bigger than anyone in this area or for that matter most places. Also if you see a deal on used equipment that looks to good it most likely is, so stay away, everyone doesn't sell equipment that is like new like I did for super low prices.
If this sound harsh it isn't meant to be. I just don't want to see you make the same mistakes again.
Happy Thanksgiving
Joe W.