"Best and least expensive" are probably not one and the same item.
Overflow boxes are not a perfect solution...especially ones that require an auxiliary pump to survive a power outage. If you are sure you want to integrate a sump, I would try to find the Aqueon overflow. It's a traditional design, which is usually the least trouble prone if made well (which theirs should be).
I would at least consider keeping the tank an "all in one" (AIO) design though. First advantage is that risk of plumbing failure goes down to zero.
Tunze's Comline series of in-tank filter equipment and skimmers can be more unobtrusive than an overflow box and can be just as handy as a sump in many ways. Check out their two "Reef Pack" systems, just for example.
Also, there are lots of hang-on skimmers like the AquaC Remora (typically <$100, used) that go on the tank just like your Penguin filter.
Technically, live rock, water flow, and a skimmer are all you should really need. (And heaters aren't that hard to hide.

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Hope this helps!
-Matt