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