Hi Keth. Personally, I wouldn't bother with the particular one you've seen, as it is more than likely a small spray gun, rather than an airbrush. Humbrol and Badger used to produce similar units. There's nothing wrong with these, but they are designed to cover larger areas, where a full-sized commercial spray gun would be too big, if you get my meaning! It will spray fine, but the coverage will be around 2 inches or more across (50mm), and you will not be able to adjust down to a finer spray pattern. This type is ideal for achieving an even, smooth finish on, for example flying models, but not paricularly suitable, overall, for the scales you will work with.
I mentioned in Trackend's 'Wellington' post about a source for inexpensive airbrushes, and a good starting point is the Badger 350, which will look similar (probably) to the one you have seen, but, depending on the nozzle fitted, will go down to about 4mm width spray pattern. Model Hobbies do a copy of this, which is fine, for around £7 I believe, and it can be used with propellant cans. An inexpensive, simple compressor, will cost you about £40, and will be more than adequate for your needs.
However, call me 'old school', but I believe in being able to walk before you can run, and think that being able to master the use of paints with a brush is a better 'education' when it comes to painting models. This helps to understand paint and how it behaves, and it doesn't take that long, with practice, to achieve various effects. The limitations are few, and it's only really when trying to achieve, for instance, mottle camouflage on Luftwaffe types, and some 'special effects', that the good old paint brush becomes really limited in its abilities.
I recently wrote a simple brush-painting guide for Karl, and Igor (Imalko) has asked if he can have a copy. I'm currently doing a bit of 'tidying' on this, and will be posting it, in PDF form, hopefully later this week. This might give you some pointers on how to achieve good results with a brush.