Hi Nord, as to when the best time to buy an airbrush is...as soon as you can afford a decent compressor I would say. If you just want to practice there are any number of cheap airbrushes (I personally have a Badger 200 single action and an unbranded dual action - both cheap airbrushes, but I have a Revell Master Class compressor).
The compressor is important though...don't get a cheap one as it will cause you issues with things like overheating, poor and inconsistent air flow, small or no storage tank and oil or water in the line (although you should have an in-line water trap on each airbrush anyway).
I actually went the little extra and fitted quick release connectors so its easy to disconnect one airbrush and connect another (if you use more than one).
At the moment if you only have 250 Euro's that will probably get you the compressor - the Revell cost me £200 last year and is still being sold at Wonderland Models (UK) for that price. On Amazon the same compressor is going for £300.
I think your best bet would be to save a bit more before you buy, or do like me and just buy bits as you can afford them as it' not just the compressor and airbrush you need to think about.
You will also need cleaning tools, cleaning fluid (I use Muc-Off bike cleaner
), in-line water trap, hoses, spare paint bottles (depending on your airbrush and painting habits) etc, etc.