I would buy two Emp 400s. I would toss the disposable Emp cartridges and buy AquaClear 500 foams to fill the filter well as mechanical media. I cut the 500 foam so it's just a bit longer than the filter well, insuring a snug fit.
You can use two blocks of foam per media well, or only one. If you use just one block of foam, you have to push it into the well just enough so that the biowheel can spin freely, but no further. If you use two blocks per well, just push then both in until they bottom out. Four AC500 foam blocks will fill a single Emp (with two cut pieces per well). They are easy to clean and will last pretty much forever. They provide good mechanical filtration, too.
I don't know that I'd mess with a Mag 350, esp. if you're trying to watch expenses. It's hard to less expensive filters than HOBs. Aquaclear 500s are also good filters (a pair would be OK for you) but I much prefer Emperors for reliabilty and for separating mechanical and biological filters.
HTH,
Jim