Prusa is top of the line for open source and very much falls into “works out of the box”. The other bed-slinger style printers like Ender are cheap but tend to have issues and are a money-sink for upgrades to get them working (lots of mods there). If you’re into hacking and modding stuff though, look into Voron printers. I’m not sure if any of the Voron versions fall into your budget, but you can buy a kit or do open-source. They’re self-built, extremely fast, customizable, and really meant for the people in the printing community who like to program and mod things.

I was in a similar position recently, but also valued OpenSource (see meel being here ;) )

I went with the Prusa Mini+ semi assembled kit.
it’s been chugging through filament for a few months.

I picked up an Ender 3 S1 Pro a month ago for what would be about 350 Euros. I hear about all the problems people have with Enders (now that I bought it) but so far it’s been a good unit. Been using it most days since I put it together, with no real issues with the unit itself. Any problems I have dealt with are just me learning to use it properly or figuring out settings for the slicer software. The Pro comes with all the upgrades already installed that people with earlier versions seem to be trying to add onto theirs. It lacks WiFi, but I started running Octoprint off an old cell phone about 2 weeks in. Can upload gcode files, start prints, and monitor the system using the phone’s camera from my pc upstairs. In time I may start running into issues, but for now I’m happy with this printer.

There is the Sovol SV07 for about 300€-ish. It comes with Klipper and should check all your boxes. I bought the SV06 Plus two months ago it works OK. You have to tighten all the screws and actually read the manual, but that’s it. If you want a guaranteed problem free OOTB experience, you have to buy Prusa (top quality control and software) or Bambulab (not really hackable but runs well) for 600-800€.

I got a Prusa (MK2s) six years ago and it’s been really, really great. Unfortunately, your budget restriction rules out the current model.

If you can get access to a printer at a friend’s house, a library, a school, a workplace, a makerspace, etc., I would suggest not buying a printer while totally new to 3D printing. Do some 3D printing for awhile. Then, on the basis of that experience, decide if you want to commit to having a good printer at a slightly higher price point.

400€ is really very restrictive. Commercial 3D printers are 10,000€ to 100,000€. Prusa’s printers are extremely good for their modest ~900€ price.

If you do decide to get a Prusa, I recommend the kit over the assembled option. It’s cheaper, and the familiarity you get with its components and construction during assembly gives you the power of fearless repair and tinkering – you’ve already assembled it once, so disassembling it and reassembling it for repair or upgrade is no big deal.