pay a labourer 30 quid a day to mix up with your paddle wheel and clean up, and increase your price slightly to cover him
on the machine side, i do this for a living and bought the cheapest half decent thing i could find, evolution twister, lasted 4 months and the trigger/switch went, bodged it and im still usin it, 100 quid. theres cheaper on ebay, around 60 quid...
i wouldnt bother spending any more than that for now mate...
problem with using a drill is u can only really use little paddles that fit a chuck, what you need if your gonna do it mechanically is a bigger paddle and theyre m14 thread, so anything that has a female m14 with take a decent paddle...
this is all assuming your knockin up upwards of a bag/bag and a half at a go..
if not, you can buy a little cheapie 'paint stirrer' and shove it in a cheapie cordless, just mix 1/2 a bag to a bag...and dont expect it to last more than a couple of months...