I thought I would share what I use to mix mashed potatoes. A wooden handle dough whip, I got it for pizza dough, but I use it all the time for mashed potatoes. Its great, and very fast, we prefer a slightly rustic texture, but you can whip potatoes to a pretty smooth texture too. The shape allows you to work right in the pot you cooked the potatoes (water removed ), it will get down in the seam/corner. I haven't tried it on really large batches but for 2-8 people it works fine.
Give it a try, $ 7 or 8 bucks and if you shop as low as $5.