The citadel one is a very very very basic suction feed one.
For a "cheap" airbrush I'd probably recommend a Harder and Steenbeck Ultra myself, it's not the cheapest on the market by a long shot (you can get badger clones from about £20), but it's easy to get the spares for, it's easy to clean and you can buy different size cups for it (so if you're doing a lot in a single go you can fit a larger cup to hold more paint).
The main thing though is to get a gravity fed double action, as the gravity feed means you can use it at a lower pressure (you're using gravity to pull the paint into the brush, as opposed to the air having to suck it up from below), and it means you can control the air and paint flow separately.
One must have is a compressor to go with whatever brush you choose, as propellant cans have little pressure control (and it varies depending on the temperature/how much is left in the can), and rapidly costs a fortune at £5-10 a can. My suggestion would be to look for a cheapish airbrush compressor, preferably with a 1-2.5l tank, they start at around £60-80, but if you think you'll do a lot then a better/branded compressor (Sparmax, H&S, Iwata, Bambi, Junair) is well worth it as sometimes the cheapies have QC issues. If you get really lucky you can sometimes get a bambi compressor cheap from dentists/schools that are refurbishing or upgrading equipment (I kick myself for not bidding on one that was local to me and went for £120, they cost about £300+ new).
Do not get a garage compressor, they're cheaper and have huge tanks, but are far too noisy.
All told you can start airbrushing for about £100, but if you will be using it a lot it may be worth paying more.
To put things in perspective I paid about £150 for my Harder and Steenbeck Evolution 2in1 FPS (it came with two needle/nozzle sets, two paint cups, an adjustable needle stop and air control valve on the brush), and about another £220 for the compressor.
I've also got a H&S Ultra as a cheaper brush which works nicely but doesn't have quite the same refinement as the Ultra.
If you're looking at doing base coats have a look at the Vallejo model air and game air range of primers, as you can get about 20+ colours and the airbrush is great for priming the plastic models, and works out far cheaper than spray can primers (largely because a 200ml bottle of airbrush primer is just the paint/primer, no propellent so it goes a very long way compared to a 500ml spray can*, and costs about £11 vs £5-10 for the can primer), as the primer is available in the colours you can potentially have the primer as your base colour saving a lot of time and money (or at least potentially saving doing a coat or two to get the right colour).
The Game Air range has some lovely pure red and blue primers, and I'm hoping they do what they did with the Model Air primers and release larger bottle versions (the normal 17ml bottles are about £2.70, but the 200ml bottles are about £11-12), as if they do that it would make doing various marines, orc vehicles and Necrons incredibly cheap and fast.
Don't forget if you're buying an airbrush you'll want cleaner (about £5-7 for a 200ml bottle that you'll use a few drops at a time), and possibly a facemask.
We're currently using a mix of Model Air, Game Air, Game colour and Citadel paints in our airbrush, but with the non airbrush paints we are thinning them with proper thinner (Vallejo do a 200ml bttle for about a tenner from memory, don't get the "white" older version, use the clear new stuff), as if you just use water as a thinner you lose a lot of the stick the paint has when its thinned.
From memory with our setup and using current citadel layer paints we thin at about 1:1 or 2:1 thinner to paint, but it takes some experimentation as it varies depending on your brush and paint.
*I think I used about 20-30ml of the Vallejo brown primer to do about 80 Skaven.