For fish you should just get the biggest tank you can fit, or at least one that you are comfortable keeping your largest fish in for 2-3 months max depending on problems.
I use a 55g
Jeabo WP 40, so I can have a full on wave blowing water out of the tank for Achillies tang or turn it down for another fish. I actually like to pre QT fish like this where as I put him in my frag tank first, got him eating good, treated with prazipro for flukes (reef safe treatment but I still wouldn't do it in DT) then transfer to the 55g where he was treated with copper. I seeded the sponge from my DT tank but the QT tank still went through a cycle. Originally I was changing water every few days, but haven't had to change for close to 2 months other then for the purpose of removing the copper. I use an ammonia alert badge since ammonia test kits won't work with copper. Only other equipment is a heater and cheap thermometer even if the temp isn't exactly right I know it's stable.
I'm actually going to probably take this tank down but only due to the fact I will be moving soon it will eventually be setup again. If there are any must have fish before the move I will do a similar setup in a 10g.
I don't but should QT corals especially since I have the means. I just dip them and put them in. Coral RX.