Hydraulic steering resevoir ?
 

Is an external power steering resevoir always needed with a full hydraulic steering system. They don't always appear to come with the full kits.

no

Guess the next question will be when is a power steering resevoir required?

I believe a reservoir is necessary. Without one, even a small leak could leave you without steering. Most manufactures sell some kind of tank.

You need one unless you like cleaning ps fluid all the time. It helps keep your level constant.

Not absolutely necessary. But, it makes it a lot easier when bleeding the system vs trying to keep PS pump reservoir filled while air is burping. If you look at most external kits from the major external steering manufacturers, they all include it.