I'm not seeing anything too alarming, except the lionfish in a community tank.

Your clingfish should be great.
Re buy v/ build, I'd say build if you have the time...and you do, if you don't push the tank. You can buy a used tank if you know for a surety that copper was never used in it. (A test is to water it up with discarded saltwater and run a strip of polyfilter in a filter): if copper is loose in the tank, the polyfilter turns azure blue. Easy to install baffles (barriers) high and low to create your own sump.
A downflow box is harder to make: you have to cut your own plexi, and you need a model, but we have a forum (The DIY or Do It Yourself forum, ) which can save you a ton of money. Things easy to DIY? Kalk reservoirs and kalk reactors, sumps, in particular. Some have made wavemakers, antibubble devices, tanks, and, ---well, just take a tour. If you can handle a saw and glue two things together at a right angle, you may find kindred souls here.
Start your tank with plenty of inverts. Avoid coral banded shrimp and arrow crabs and anemones and, MOO (my opinion only) cucumbers, toxic little devils. DO NOT kill off your bristleworms. Just don't try to pet them.
Here's a tip: type the name of your fish in the panel above marked "Live Search" and you will likely find out how big it gets, what it eats, and where it's from.
DO NOT assume that a fish with an adult size of 18" will only grow to fit your tank: it will just die before it gets too much bigger, and that's just not right.
How fast do they grow/do you have to come up with a mega-tank? The bigger the fish, the faster/bigger the growth spurt, given decent conditions. You can have a fish that increases inches in a year.
And remember tangs and angels are oxygen hogs...Somebody comes online in tears and says their tang and angel are suddenly dead with no apparent cause, and the first thing I ask is: How's your oxygenation?---because the fact it was the tang and angel that checked out together usually means that's what's wrong. Water temp rises a critical bit, limiting ability of tank to supply oxygen, and they're gone.
That's one other reason we say: big tank for tangs and angels.