I set up a 240 and have some thoughts. I don't know if they'll apply or not --
1) I've learned that I should have bought good stuff first rather than upgrading and upgrading again.
2) Do your lighting research and figure out how you want to light the tank. My "best bet" right now for metal halide would be 3 really good reflectors and 20K radiums. I have 4 over my tank right now, mostly as a test, and it's almost too much light, if you can have such a thing.
3) I like simple sump designs. 3 compartments work well for me, fuge, skimmer, then return section. Two $10 baffles and a $40 used acrylic 50 gallon tank and I'm done.
4) Be sure to think about humidity in the room behind the tank. I don't have this situation, but have read about it many times.
5) Plan your rockwork. I started off with what turned out to be a wall of rock and there are much more creative, more appealing rockwork designs. Think about coves and caves and arches and outcroppings and open areas and things that make you have to look behind or look at the tank from a different angle to see something rewarding. I'm definitely incorporating this in my new tank.
6) Read about flow and plan your flow accordingly. The difference between creating flow in a 50 gallon tank where you can throw in a single Koralia 4 and have the whole tank going nuts and a 240 gallon system is not trivial. I have three Tunze 6100's in my 240 and wish I had a 4th. Investigate closed loop. I'm going to try a closed loop on my upcoming 210 and I have high hopes for it.
7) It's taken me more than a few months to learn to let go of the old tanks and focus on one.
8) If you can go to a 300 and have it be 30"wide, go for it.
Those are my thoughts. I haven't been doing this that long, but I've put a tremendous amount of time into the hobby. I'm really excited about my next tank and really think I have figured out what will work for me. Hopefully at least one of these points gets you thinking about something that will be useful to you.
Ron