For your high shank industrial, you can get a Big Foot darning foot or you can get a Juki high shank embroidery hopping foot. I have both and they both work just fine. The Big Foot (plastic) costs about $28 and the Juki (metal) about $40. The Juki is a lot smoother and gives you better visibility than the Big Foot, but either one will do the job.
For your 15, sew-classic.com has one of the best darning feet I've ever tried (I think I have about 10 different kinds, including some antique "stripper" and darning feet and springs) - the one that works the smoothest and gives you the best visibility costs $3. :)
You don't have to drop or cover the feed dogs, but it gives you one less thing to fight if you can. If you can't, just set your stitch length to zero so that the feed dogs aren't pulling in one direction while you're pulling in another.