Congrats on the build and the house. I am pretty much in the same boat, looking for a 300G Deep Dimension or something comparable, will be upgrading from a 150G DD.
Anyways, to answer some of your concerns here are my thoughts:
Return - if you plan on pumping water up from the basement, it is inevitable to not have an external pump IMO. The only thing that I think could come close to having enough power is maybe one of those Ecotech Pumps the L2 specifically unless there's something similar. At the same time I don't think submersible pumps have the strength for head pressure that you'll have to account for. I personally have a basement/living room setup right now and I have a Reeflo Hammerhead Gold to push water through approximately 20 horizontal feet and 10 vertical feet of 3/4" PVC x2. I also have a client who has a Blueline 100 HD that pushes water through probably 20-25' of 3/4" PVC at height of about 15' also x2. I was thinking about pumps like the Ecotechs but I would probably use one for each return line.
Skimmer - for a budget friendly skimmer I think a Reef Octopus would be ideal. There are a bunch of models to choose from and I'm sure you can find a used one in decent condition on any of the hobby forums. I personally use an old SWC Xtreme 300 which even with a heavy bioload is apparently rate for 365G, if you could find a used one, I'd highly recommend it with the taller neck skimmer cup.
Sump - 150 is nice but it will be hard to separate anything. I would suggest maybe a 75G or 90G that you could make your own baffles for and arrange any which way you want. I was toying with the idea of a 90G just because of the increased height that would catch any water that could possibly backflow in the event of a power outage but honestly, I think a 75 is tall enough.
ATO - will run off a new ApexEL so will need a float and breakout box. I'm not sure how you plan on topping off but I find it very efficient to have your RO run straight into your tank and allow your Apex to control a solenoid to the feed water of your RO. This way you don't need a freshwater reseirvoir.
Circulation - personally, I think MPs are overrated for their price. I have been a huge fan of Gyres and although they carry a decent price tag themselves, I think that one or two of the larger Gyres should give you enough flow couples with your return lines. Another budget friendly wavemaker are the Jebao RW series. I think 2 gyres on each side along with a couple RWs that you could mount to the inside of your overflow box if you have the space for the magnet. The nice thing about the Gyres is that you can mount the vertically if you wanted to give you a nice flow around your overflow box.
I hope this helps. I've been preparing for a very similar build and should be closing on my place by the end of the month. My only thing is that I don't have a tank yet. Good thing is most of my equipment can pretty much be reused and I already got a ton of new rock which filled a 55G tank and is currently cycling then off to build my sump with this 90G I picked up.
Good luck with your new place and new tank.
. I am not opposed to circulation coming from a side but would prefer from the ends.
