1) You should never "slap" a VTEC head onto a non-VTEC block. That is essentially what early LS-VTEC setups were, and that is why they had so many problems associated with them. That said, $300 is a very good price for a complete P72 head. A B16 P30 might flow slightly better, but it's not that much of a difference. A P30 head will also yield a lower C/R, which may or may not be what you're looking for depending on what you're building.

2) The GSR manifold will not fit your B18B head, and if you end up getting the GSR head, I would ditch the OEM manifold and get a Skunk manifold.