I got a real old Milwaukee D-handle drill that I got out of my grandfathers basement many years ago....its real old and hade it just laying around,,,,this is what I use it for a lot and I cant kill it,,,,,and it don't even get hot !!!
I have a couple of the Ridgid D-handle or Spade handle drills that I have been using for thinset/cement mixing for years. I have also used makita's and dewalts version with the same results. These drills are designed with mixing in mind and will hold up to it. When I was first starting out, I had a cheap black and decker 1/2 drill and it started on fire. Then I bought a nice 1/2" milwaukee hammer drill and it ended with lots of smoke. I finally realized if I was going to be mixing a lot of mortar I might as well get a real mixing drill. You can find the ridgid factory reconditioned for about $100. I originally bought mine because they came with cases and the makita and dewalt didn't. I don't think any come with cases now.
Be careful mixing grout and thinset with drills not designed for mixing as they will spin too fast and entrain too much air in the mortar and damage the mortar in other ways(especially the grout) and it will weaken it and cause bubbles in grout.
I wound up going over to Harbor Freight (have to admit my first time in the store) and buying their low-speed drill for $49.
Mixed at least 12 full bags of thinset over a couple of weeks, worked fine, got me through that job. It's still working, anything I get out of it in the future is gravy.