1. How many quarts of color (black, single-stage) should I get?
If exterior only, 2 quarts will get it. Jambs too, then get a gallon. I mix a gallon when doing an all over with jambs and still have remaining paint. With 75% spray out overlap it will cover quickly.
2. What kind of prep (other than masking and stripping stickers / remaining wax from the paint) do I need to do?
Red scotch brite wash twice. Like your scrubbing a pan with dried up shit in it.
3. Would multiple coats be more durable? (seems obvious, but thought should ask)
Depends actually. Multiple coats will create thicker paint, giving it a better opportunity to peel and chip down the road.
4a. How long before it's safe to get it muddy?
4b. How long should the paint cure before I hit it with a wax?
I dont cut and buff single stage until 72 hours after its been sprayed. So I would keep it in that range to make sure its fully cured. If not it will gum up when trying to buff. I know washing and buffing are 2 different things. I have washed fresh single stage within 6 hours of force dry.
I dont wax single stage either, but I would wait 30 days. Typically with single stage, it just gets polished every so often. It will dull down faster then base/clear will. Im not talking month, but over the years.