In my opinion, it is worth buying a more professional system. The airbrush system from revell was quickly defective after moderate use... it's better to invest a little more, but only once.
Yes, the Citadel colours from Games Workshop can also be used. Citadel then has two variants of colours: - Air is already diluted and usually has the finer pigments. (For example, the White Scars White in the normal base variant is extremely flaky - the Air variant is less opaque but much more consistent). - Base/layer variants can also be used. However, make sure that you shake and mix them very well (with a mixing ball!). For both colour variants, dilute until they can be sprayed well. Undiluted colours (also the Air) clog the gun.
