Hire one, unless you want the shed to be half-full as soon as you've
finished building it.

Cement mixers are surprisingly cheap to hire, and I've never bought
one for the clutter reason.

But if it's going to be a slow job rather than getting it over in one
big bang, buying may be more economic.

For concrete, you need a reasonably substantial machine. The little
diy ones are ok for cement, but struggle with the larger (and
lumpier) batches you're likely to want with concrete.

Having tried various machines, the belle minimix is well worth what it
costs:

http://www.screwfix.com/prods/13174/...ete-Mixer-240V