1. What kind of filters, regulators, valves, and plugs are required in the
air pressure line from the comressor?

This can be quite involved or quite simple. You can use iron pipe and plumb
in traps and drain valves, or just run a hose from the compressor to your
gun. Basically, you should have a particulate & coalescing (removes oil)
filter not too far from the compressor. You'll want a moisture trap farther
on down the line (works better once the air has cooled a bit, and perhaps a
small button unit that attaches directly to the gun, or close to it.

3. What is the right needle size for oil based polyurethane?

It depends on the gun and the brand of finish you're using. The viscosity of
the finish can be measured with a viscosity cup. The viscosity cups have
handles and a of hole of a certain size at the bottom. You dip the cup in
the finish, and count how long it takes to drain from the cup. A gun/needle
combination would have a recommended range of finish viscosity, for example
20-25 seconds through a Ford # 4 cup. You would thin the finish, if needed,
to achieve the recommended timing range.

8. Is pressure feed (or pressure-assist) required for good woodworking
applications?

Pressure feed guns can spray a wider range of finish viscosities. You can
get gravity and siphon feed pressure attachments, but most of the time what
is being referred to is a remote pressure pot of a gallon or more that
supplies finish to the gun under pressure. There are several advantages;
there is no attached cup therefore the gun is small and compact, you can
spray in any direction, even upside down, and you don't have to refill every
few minutes. Pressure feed is great when you have a lot of spraying to do
and will be using the same finish over an extended period of time. The
disadvantages, other than the extra cost, is that cleaning up and changing
finishes is more time consuming. I have gravity feed guns and find them the
most practical for my needs.