I use a 4gb raspberry pi 4, previously used a 8gb 8th gen i5 laptop, that was overkill, and one of these, I don't have access to a roof or balcony, so just use a small aeriel on an indoor windowsill, but still get about a 50mile radius reception
I used a Raspberry Pi Zero 2W and this kit from ADS-B Exchange, which I bought on Amazon (US) for $40. Total cost was under $100, even with egregious Pi prices. I don't know how much more expensive things would be on your side of the Atlantic, but I'm guessing it's definitely doable for less than 350€ even if you don't want to build anything yourself.
Better SDR, Better coax, a 1090 LNA?
What antenna?
Local RF?
Are they SDR with 1090 filters?
Example:
https://www.amazon.com/ADSBexchange-com-RTL2832U-Antenna-Software-Industrial/dp/B09F2ND4R6/
https://www.amazon.com/ADSBexchange-5-5dBi-N-Type-Female-Antenna/dp/B089Q4BVCB/
Those SDR will burn up. RB24 ones run very hot due to small PCB I replaced mine with ADSBx metal cased ones after RB24 ones died in under a year.
You might want to buy a Pi case with a case or one of those metal ones. They work great, Pi 4 get hot.
https://www.amazon.com/ADSBexchange-com-RTL2832U-Antenna-Software-Industrial/dp/B09F2ND4R6/
​
https://www.amazon.com/ADSBexchange-com-Orange-R860-RTL2832U-TCXO/dp/B09NJWMY56/
​
Also for operating system, the ADSBx image is a fantastic all in one with wifi web gui setup.
https://www.adsbexchange.com/how-to-feed/adsbx-custom-pi-image/
.
I had 2 Radarbox sticks fail in under 12 months. The PCB is too small to properly dissipate heat, that seems to be consensus.
Any rtlsdr-based dongle with a r820t tuner. You can use a lot of SDR, the really cheap ones might not be RTL. AirSpy or Stratux SDR are other options.
ASDBexchange.com has put out SDRs to support the community.
https://www.amazon.com/ADSBexchange-com-RTL2832U-Antenna-Software-Industrial/dp/B09F2ND4R6/
I just installed their newest image and it's freaking amazingly simple to configure now. You can feed whoever from it, just follow the guides for the clients. FR24 is problematic and will break things if you aren't careful. The also force update and break things now and then.
All the information you will ever need at:
https://github.com/wiedehopf/adsb-wiki/wiki/Raspbian-Lite:-ADS-B-receiver