Manifold is a peice of something(typically metal, but can be plastic and such) whos sole job is to channel SOMETHING from point a to point b. Such as your INTAKE manifold on your car:
It takess the air in at point A, and channels it through 4,5,6 or 8 holes.
Basically think of it as a divider, takes one source of air, and divides it among a set number of paths.
As far as buying solenoids.. look around and buy GOOD solenoids. Nothing is worse then building the system with 50 solenoids, and have 10 of them fail.