To your original question about buying or building, you should make that decision based on your wallet and your time. I think you can get the flow you desire in either choice with the right setup. Also, the PVC
one will probably be louder than the box you're looking at. I don't know this for a fact, just from what I have read on the web about other builds.
I'll throw in a comment as a recent DIYer and some lessons learned. I'm still in the process of refining mine or trying to make some rework decisions. This is just from my POV so take it with a grain of salt. I have a video and some other stuff as well as a overflow thread here in this section. Benjamin's 55g Malawi Cichlid Build
I had a check valve leaking air whihc was killing one of my intakes too... another headache with DIY.
The thing I dislike most about my setup right now is trying to constantly match my overflow and my return. I'm always adjusting the dang valves trying to evenly match them and keep the water level in the sump above the pump intake as well as keep the water in the main tank high enough to quite the overflow. Because of my box design in the tank and the small design of the sump it's battle. Most likely my design flaw.
However, if I do turn off the valves to do anything, I have to adjust everything all over again. If I redid it I think I would do 2 vavles. 1 to act as a restriction to tweak my flow and the other as a straight on and off. That way I'm not trying to find that sweet spot forever after maintenance.
The other thing is where you want your water line and how much space you have in your sump. When the power cuts off you need space for the water from the return overflow PLUS the water in the pipes to fit in the sump. Again, with my design there isn't a whole lot of room for error. It's right at the rim when the power cuts out. This is also something I'm going to try and adjust by lowering my overflow edge and being able to lower the water in my tank some.