Would it be stupid buying a 900 EUR guitar along with a 300 EUR amp?, (the fender mustang 3)
What guitar are you talking about? You can get the Jet city JCA20 combo with that amp budget. All tube. Definitely better than the mustang.
As long as the amp covers your needs, not at all. And with the amount of good, no frills, low wattage tube amps on the market at very reasonable prices, it's not a bad plan.
There are some 900 Euro guitars that are stupid all on their own. there are plenty of 300 euro amps in the same category. It all depends on what they are, and what your needs are.