Hey Kay, I've heard both good and bad about Hi Flow water pumps, but more good than bad. Some say that the higher flow can cause pockets in the head where the water may get stuck for a prolonged time and cause a hot spot. I don't know how true it is, but I've always used Hi Flo pumps without a problem. To answer your questions:
1. They are supposed to.
2. Either one is suitable so go with your choice.
3. I use cheap ones out of Summit, or the cheapest (new) I can find at the time. So any extra cost is too much in my mind (said it before, cheap bastard).
4. I would go with the Hi-Flo, it really should be the better of the two.