1. If your going to build the body and neck, you need the templates first. Get your templates, then see what size wood you need according to your templates.
2. Depends on what kind of wood your looking for. Mahogany and bubigna get large enough for 1 piece bodies, walnut and alder are not that wide.
3. the scarf joint doesnt make a difference in wood size for the neck. Its still the same length either way. The only way it would make it easier is if you scared a shorter piece, and then used a different piece for the headstock.
4. Wood prices vary EXTREMELY from state to state. You have to find out what prices are like around your area.
5. Veneer on a set neck: Are you talking about putting it on a guitar that already has a neck glued in, or just a set neck guitar that you would build? If its before you glue the neck in, its no different, its fine. It would be quite hard to put a veneer on a guitar that already has a neck glued in.