Your quantity of shingles will depend on the pitch of the roof. Shingles are typically sold by the "square", which equates to 100sf.
Manufactured roof trusses will likely be easier and more economical in most cases. Site-built trusses are uncommon in today's construction market, as they are not quality-controlled like manufactured trusses are. Plus, you'd have to pay an engineer to design and then evaluate their construction.