Again, thanks all for the input.
@jackrae: Nice link. Also, significantly different from what else I've read in re. media load, and what influences speed. Yes, I'm 99% sure I will be running more slowly. It will take some trial & error, but fortunately for me, I don't think that beyond a certain point, particle size will be of much importance.
@liudr: It's justjed ... Baaaaaahahahahahahaah! I've been waiting for the chance to say that! ]
Interesting about those stickers. That + what Grumpy_Mike said seems to rule them out with certainty. (Not that I thought anything uncertain about advice from GM.)
@Udo Klein & jackrae: I hadn't considered optical, simply because I am concerned about needing to design to eliminate stray light readings. No doubt this can be done successfully, but a magnetic system seems to me to be more tolerant of play in positioning. Well, really, I hadn't given it much thought at all, in part because typically, in tach applications, I see hall sensors used.
@dc42: I like that part. Less expensive than others I have looked at, and it will make no difference to me if I have to do alternating poles. If I use an ISR in mode CHANGE, I won't even care which way it flipped.