You are very wise to be thinking about that aspect. It is much more important than anything else to long term keeping of a big tank.
Some sort of drain is essential. I'm managing to live without a floor drain, but I had to raise my sumps in order to connect to a wall drain. It is all about height.
Sink with hot/cold water and drain. Essential.
Storage tank for purified water. Mixing tank. Need to valve from the storage tank to the mixing tank to make up new SW.
Think about how much water you want to change out to do a water change. That means capturing some measurable amount, dumping that directly to a drain, refilling that measured amount directly from your mixer using a pump.
Don't cram your sumps underneat the tank if you can help it. Very useful to be able to keep the sump(s) clean and access is everything.
My fish room is only about 8' x 4' and sits directly behind my 6 foot tank.
Here's what my fish room looks like.
RO/DI feeds the 100gallon storage tank on the upper right. It is elevated to gravity feed the evaporative replacement water, as well as gravity feed water into the mixer (below it). Municipal water pressure and gravity are still free...use them to deliver water to different places.
You can see the sink. There is actually a 50 gallon sump immediately to the left of the sink that functions as a water change sump. I catch 50 gallons, dump 50 gallons, replace it with 50 gallons of new SW, then return this sump to active flow in the system.
The main sump is in the lower left of the photo. It protrudes out from under the display and allows me to put a very tall skimmer in the sump.. (I've since installed a larger skimmer than the one shown in this photo)
Finally, I left the back of the tank clear so that I could see into the tank when I'm back there. I would REALLY recommend this since I spend so much time back there it is nice to see into the tank while I'm there.
Good luck. Hope this helps get you thinking.