The idea of a manifold is to provide the number of outlets you need for your set up without causing a reduction in pressure from one end of it to the other. If you imagine a manifold made of only airline tubing, you can see that there would not be as much air pressure at the far end as there is at the end near the pump. But as long as the PVC tubing is over an inch in diameter, there will be almost no reduction in pressure from end to end. PVC is cheap and easy to cut and since it is easier to put valves into a 2 inch PVC than a 1 inch PVC, the larger is usually chosen.
There are a number of different valves you can use. Brass ones are the most expensive and last forever, well almost. Other types of metal ones and plastic ones are available. All require you to drill a hole in the PVC and insert the valve. That way you can put the valves where you need them. I put mine in after I have hung the PVC above the aquariums or mounting it to the rack.
Connecting the manifold to the air pump requires a hose, some hose clamps, and fittings to connect the hose to the PVC. All the fittings are available at most Lowes or Home Depots.
As for the air pump, the bigger the pump the better and liner air pumps are great because they are very quiet, they produce the amount of air you need (no need to bleed of excess air), and should require no maintenance except for making sure the air filter is clean. Dirt getting into the air pump is what causes them to wear out and need new parts or replacement. You can enhance the air filter using quilt batting.
If your air pump is not producing enough air for your set up, you can tell because you will be constantly adjusting air valves to get enough air to all tanks.