The fitting on the end of the airbrush hose is an air and pressure regulator for use with propellant cans. This means you can 'turn off' the air supply when required, or when fitting/removing the regulator from the can. The airbrush itself is fine, I've used a similar model for about twenty years until recently. If you look at the tip, probably on the underside, there should be a finely-engraved letter, either 'F', 'M', or 'H'. This is the nozzle size, Fine, Medium, and Heavy. The 'Fine' nozzle will allow a spray pattern down to about 4 or 5mm and up to about 20mm, whilst the medium is about 9mm to 30mm; it's unlikely to find a 'Heavy' nozzle these days. The small black tubs are, as jan said, paint cups for a different airbrush.
Using propellant cans is ok, but they do suffer from a fairly quick drop in pressure, which a quick shake can sometimes cure, and can have a tendency to pulse, causing 'spitting'. Also, they tend to work out expensie, at around £7 or more per can. On a model the size of the 1/48th scale Wellington, you would probably need two to three cans to complete the job, as the remaining 15% or so of air in each can will possibly not deliver controlled pressure for spraying, and will be needed for cleaning the air brush and practice.
If you decide to use the airbrush, I strongly recommend a lot of practice first, possibly using kiddies water colours or ink, to save on paint costs. Get used to the control and 'feel' of the brush, and the effects of the trigger.
Considering the cost of propellant cans used per model built, it is well worth considering a simple, inexpensive compressor. You can obtain one of these for around £40, as you don't need a 'professional', all singing all dancing unit to meet your requirements. A top - quality compressor will set you back somewhere in the region of £90 to £200, but remember, these are designed for protracted and hard use in such places as design studios, graphic artists studios etc, and are rwally an overkill for the average modelling use.
A couple of outlets for inexpensive compressors are Model Hobbies, and Everythingairbrush.com
I hope this helps a bit Lee, but be careful - once the modelling bug bites, it never heals!