I got a relatively cheap one at Wal-Mart last year (metal, single wheel) and it seems to work fine.
As for the one-wheel vs. two-wheel: The tradeoff is stability (two-wheel) vs. manuverability (one-wheel). single-wheels are easy to tip if you're not careful, but they're also the only kind you can roll up a 2x6 and dump into a footing...
As for the best way to mix concrete -- I'd say rent an electric mixer. I'm actually going to be doing this in a few hours to pour 7 deck footing tubes... I did it the manual wheelbarrow way a couple years ago for a buddy's deck and it wasn't much fun.
-Tim