With natural New York slate readily available from local suppliers plus a definite trend in our area towards natural construction materials being requested, it has become a specialty of ours. Some specialized equipment is needed (no, it is not the same as tile for cutting), and most importantly is some experienced roofers to do a decent job of it. We have picked up a lot of work fixing botched jobs from fly by night companies that tell a homeowner they can do a slate roof and have no idea what they are getting themselves into.
First thing you will need to do is get some better hydraulics for your ladder hoists because those cheap hoists for asphalt shingles are not going to do the job. The part that is hard to get the homeowner to swallow is the amount of overage you need to order in shingles. While the slight chipping on an edge is part of what gives slate roofs the natural look, any type of cracks at all need to be discarded and you are bound to break shingles aplenty while cutting them.
Tile saws are often use, but you are better going with wet stone saws for the decorative shingles. The thick shingles are easier to work with in some ways but definitely need a stone saw. The other issue is the time factor. Aside from the individual shingles, there is a few seconds lost inspecting each shingle and sorting shingles. Even a good supplier will have off shades mixed in so selling the natural look of various shades up front is a good idea. If they want all exact match there is a lot of waste and the shingles need extra sorting to ensure none “stick out” from the ground view.