Mark always answers. Not sure how he keeps up, but he does.
You spoke of having extra space for future stuff? Are you planning on a refugium? I built the same basic sump and use the one end as the fuge, the middle is where most of the water enters the sump and the other end as a return section. Just my opinion, if you are planning on a fuge (highly recommended), do it now. No reason to wait, it's just another bag or two of substrate. That way you get all of your plumbing done and get the added volume right off the bat.
Some things to consider as you build your sump: 1)Incorporate a chamber for RO
water. This often gets overlooked and you end up with a bucket or jug sitting next to the tank. 2)Make provisions for a float switch to control an auto top off system. You will understand the need for this in the first week. 3) Choose your lighting fixture for the fuge and build a bracket for it. It's easy to get your plumbing done and realize you forgot about how to mount your light (or so they tell me lol). 4) Think about making the baffle wall between your fuge and the rest of the sump out of something other than clear acrylic, like maybe black. I did not do this on mine, but the next one will have it for sure. There is no reason to have light in any part of your sump other than the fuge. Light just makes stuff grow and give you things to clean out. The opaque baffle wall will greatly reduce the amount of light getting into the rest of the sump.
As for the MarcoRocks. That rubble also makes for great mountings for frags when you get to that point.
For me, as far as this hobby goes, there is nothing more fun than a new build. Especially when you get to make your own stuff. Have fun!