If you've never constructed a retaining wall of any kind before, then it might be best to deal with materials that are easy to work with. As a poster above noted, flatter stones, rather than rounded stones, would be relatively easy to deal with, but even there you might want to consult with the folks you buy them from. Look around your area at the various nurseries and stone yards where stone is available. Usually its sold on pallets and can be delivered (for an additional fee, of course). As another poster indicated, type of stone is a choice you'll have to make on your own with regard to look and color. The right or wrong type of stone is difficult to specify on an internet setting, not knowing where you live or what types of stone might be available in your area. Stone type should be based on look you want, color you like, relative ease to work with, and weight of the individual stones (remember you have to pick these things up and place them and turn them and move them around until they fit just where you want them. With regard to the weeps, you really only need weeps in a stone wall if you're going to mortar the stones together. If the wall will be dry set, then the spaces between the stone will be sufficient to allow ground water to drain through the wall. If you are going to set the stones in mortar, then you will have to set weeps between the stones. But on a 15 foot wall, you should need no more than 2, spaced as equally as you can make them, about 4-6 inches above the grade on the low side of the wall. The gravel behind the wall is very important, as well as under the first layer of stones. Six inch minimum (compacted) under the first layer, and 6 inch minimum behind the wall. A full foot of gravel behind the wall would be better, and carry that gravel up to about 6 inches below the final grade behind the wall. As for the existing wall, if you're going to go to the effort to replace it, then remove it completely. You might even be able to salvage some of the old blocks for reuse elsewhere.

Building a dry stacked wall to run along a driveway. It will only be 20" high but it's on an incline. Most of what I read says to create steps when building on an incline but this is 200' long and about 25% incline so it would be stepped almost every 5 feet to maintain level and still have 20" hieight. Is it required to step or can a dry stacked low wall just sit in the natural incline?

Wow, what a project! Not an expert on retaining walls, but I'd probably be googling some of the questions you asked or visiting engineering/landscape forums to see what others might have to say. Best of luck. A link that might be useful: www.paversearch.com/build-retaining-wall-faqs.htm

I thought about the dry stack for our rock work, but I was told by multiple sources that it can cost almost twice as much because of the amount of materials and labor. I love how it looks, though. If I were you, I would get an estimate both ways for the whole project and go with the dry stack if you feel it's affordable. I don't know about your retaining wall. It sounds to me like it would be better to have the rock work all the same, but it might depend on how it's positioned, how far it is from the house, etc.