"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. :))
Hope this helps!
-Matt