I recently bought this treadmill for working, mostly because it was cheap:
Sunny Walkstation (out of stock at Amazon right now, but probably available elsewhere). It's been fine for the purpose except it's a bit narrow and sometimes when my head is turned to look at the far end of my monitor, I drift in that direction and hit the edge. Before I got it, I looked at a folding treadmill by WalkingPad but didn't want to spend the extra money. That was a mistake. One of the reviews on the folding treadmill said something like, "save yourself the trouble and just buy a folding one from the beginning." I would agree with that. It's nice to be able to sit down once in a while and pulling a full length treadmill out of the way is a bit of a hassle, especially in smaller spaces. Tipping it sideways may cause the tread to go off track, so folding would be nice and definitely worth the extra money.
As for using it, I like it and haven't found walking and typing to be a problem. I go at maybe 1.5 mph; slower than that and I start tripping over my own feet. I can go several hours at a time with no issue and concentrate at least as well when walking. I find that when I get deep into data analysis, I can even assume my usual high concentration lean with my elbow on the desk, hand under my chin, staring at my screen from 6 inches away and still continue walking. I have post-thrombotic syndrome in one leg (from blood clots), which makes my lower leg swell when I sit too long. The treadmill has put an end to that. It has not been a magic weight loss solution, though :)