I get your dilemma. I love the dimensions of the SR series, so went and got the sr60. When it arrived to my place, I realized that I shouldn't have second guessed the length of the tank. So I returned it and upgraded right away to the sr-80. Now that I have the sr80, I'm satisfied, but not over the moon. I should have gotten the 100ext as they have the same dimensions minus that it's slightly taller by 3". The biggest advantage would be a simple sump and plumbing set-up while getting to use the full 24' depth. That said, where I am placing the tank, I did not want to much weight and avoid putting support beams in the basement to support it. So in the end, satisfied, but one day I will go bigger but will keep the height under 19-20"s for sure. I hated the maintenance on my previous tank that had a 30" height. It was also an AIO, so I am used to dealing with limited space, but frankly, the sr-80, there is so much sump space that maintenance will be a breeze and I will likely run it even without a skimmer. The 48" length, height and depth combo will make lighting a relatively cheaper endeavor as well. Flow is less problematic and the equipment it comes with sets you up for success. Think of water changes to, the actual display volume of the sr80 is closer to 50 gallons. So a 5 gallon bucket water change is 10%. very easy, less costly on salt and rodi, and doing a 50% water change if ever needed is also easily done.
To answer your questions more directly, you can cut out the back sump section and it will just void warranty, but not the integrity of the tank. So you can add sump. If you choose to use the aio sump as is, then very easy to store a top off container down below, there is plenty of space. That said, the door configuration for the APS stand is silly to me as the way they make it, the doors create 3 small access points. I trimmed one of the doors and flipped it around so now I have one small access and 1 large access point. For what skimmer, there are many options, Marine Depot did a youtube video "Will it fit". . And your last question, no doubt its quality enough for a mixed reef. A shallow lagoon, bonsai style is what I will be aiming for. Think of a top down view as your goal, as this tank was designed for this!
Hope this helps. I have been scouring reef2reef to see other sr80's and there aren't many, but mine sitting in my living room has me getting more and more excited to get it up and running as every day goes by. (also glad I traded my sr60 for the 80!)
