I wondered if it may be possible to produce a building using resources in hand and in warehouse?
Yes. From the FAQ: "Can a worker always use the resources on its hexagon? As a general principle, a Worker can always use the resources in hand or on its hexagon. It is also possible to combine them."
Also, can a newly built building be used in the same turn by another worker? Either yours or the enemy's?
Yes. Bottom of page 47: "Excepting the player’s Tower, Constructions are common property. Any player can take advantage of them: who built them and where they are placed doesn’t matter."