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Go for it Rob! At that price, it's just over £10 for the brush at UK prices. I've just paid £30 for mine, which I thought was very cheap! As I said, for such a low price, and with good reviews, even if it's totally knackered after 12 months use, it's worth the money.
With a dual action brush, pressing down on the trigger controls the air flow, which is constant. Pressure is adjusted by the pressure gauge tap on the compressor.
Pulling back on the trigger controls the paint flow. The further back the trigger goes, the more the paint flows! All the way back will give the widest, and heaviest spray pattern, and moving the brush further from the job will increase the pattern width, but progressively lessen the weight of paint.
For fine line work, the trigger is moved back only a fraction and, the closer to the job, the finer the line. BUT - this takes a lot of practice, and normally a realtively low pressure, around 15 psi. If the brush has a .25 nozzle or smaller, you should be able to achieve fine lines of 1mm or perhaps less, with practice.
An hour or two practice and a tin of paint is worth more than a few minutes of 'real' spraying and a ruined job!