Build the tank that fits your budget and space. Bigger is always better for many reasons, stability, access, design options/layout, ease of hiding equipment, inhabitant selection/health/compatibility, etc.
You need to decide if you want brackish or salt, they are different, the inhabitants are different, requirements are different... you won't keep many coral alive or happy in brackish water.
As mentioned, you have to decide if you want aio or a sump... sumps have many benefits, most notably increased water volume.
My current tank is a 35g cube (which I very much still consider a nano) and I have custom built a 25g sump which only ever holds 15g.
Tanks can be built up on many budgets...I did mine as tight as I could see fit, and still spent about 75$ a gallon... the only things I bought new were the return pump, heaters, sand, plumbing and the tank itself...everything else is either used or DIY. Used skimmer, used diy lights, used wave pumps, used Apex, used dry rock, reactors, dosing pump, etc...I custom made the overflow, sump, stand. There is only so much you can save in this hobby, and generally it is only on the stuff, that in the grand scheme, are the cheapest components.
Don't buy cheap components to save, as it will cost you more just to buy the proper ones down the road... that said you also don't need to buy 'top of the line' equipment either...you get what you pay for, sometimes that is just a name...I can't speak to the likes of Royal Exclusive, or Abyzz, Bubble King, their reputations precede them, though I can't justify the cost. I have had jebao wave pumps well outlast my vortech pumps and my maxspect gyre pumps. I will with absolute certainty say, buy good heaters. And yes, heaters. It is far better to heat your tank with two slightly undersized heaters than one big one... when heaters fail, they generally fail ON, with one heater of adequate size, this could mean your tank reaches 90+ degrees in a matter of hours. With two heaters, it is much less likely the tank will over heat. Having a temperature controller is an essential piece of equipment in either case.
I wouldn't cheap out on salt either, it really comes down to what you want to keep and how much you want to actively control your parameters...I hate doing water changes, even though they take a total of 7 minutes to change 10g. I just hate doing them...more then that I hated having to constantly check parameters, adjust this adjust that, blah blah blah...I found going with better salt and very simple dosing regimen of AFR (All For Reef) I am able to keep everything exactly where I like it which is a close to NSW as I can. I can do water changes every 3 months or so, and nothing ever changes, calcium rides smooth at 425, alk right at 9dkh, mag at 1300 etc.
Wait, what was the question again