Cool, glad you like them too
There aren't many slim / HTPC style ITX cases that take full size components around, most tend to be boxy which I'm not keen on personally, or just ridiculously expensive.
For a budget monitor it's always a bit of a gamble but looking around I'd probably go for either the BenQ BL2480 (24" €120) or BL2780 (27" €138 warehouse deal):
BenQ BL2480 60, 45 cm (23, 8 Zoll) Monitor (VGA, HDMI, DP, FHD) schwarz - Kostenloser Versand ab 29€. Jetzt bei Amazon.de bestellen!
They don't have an adjustable stand, which you can get with the T models (e.g. BL2840T), but it's an extra €20 and 3kg of weight which it sounds like you might not want. Other than that, I can't find a detailed review but BenQ are a good brand and these have FHD IPS panels with true 8 bit colour and flicker free tech, which is about as good as you can expect at this price point (a lot are 6 bit + FRC and QHD costs a fair bit more).
Anyway now you've asked for it - onto the PC!
On the CPU front it's very close, both Intel and AMD have good value offerings, but in this case I'm going to recommend AMD because:
1. With mini-ITX providing less motherboard options and Intel's locking of lower end CPUs, you need a more expensive motherboard to compensate, which adds ~€50 to build cost
2. The AMD produces less heat for equivalent performance (due to 7nm tech) which could be useful in such a compact case
CPU: AMD Ryzen 3600X @ €189.47 (warehouse)
Great 6c/12t CPU and an amazon warehouse deal worth having... it's rare to get any discount on the new Ryzens and this 3600X costs the same as a standard 3600 but comes with a better cooler (which is bigger, but still fits in the RVZ01B-E) and higher clocks.
Motherboard: MSI B450I Gaming Plus AC @ €111.80 (warehouse)
MSI B450I Gaming Plus AC, Sockel AM4, DDR4, HDMI, DisplayPort, 1x M.2, 6x USB 3.1 Gen1 Mini-ITX Mainboard - Kostenloser Versand ab 29€. Jetzt bei Amazon.de bestellen!
Great board at this price, dependent on a warehouse deal tbh (though there are several others for ~€120 which seems fine too)... There are others but they tend to be expensive, like €200 which puts another €50 on the Intel equivalent. Also looking at the QVL memory compatibility lists, the memory controllers on ITX boards seem finicky in general (both Intel and AMD) and finding compatible RAM is a pain. Overall, this is a great board for that money... it has Wifi and NVMe, will make the most of the chip and importantly supports good RAM found on amazon.
RAM: HyperX Predator HX432C16PB3K2/16 @ €56:
Compatible with the above motherboard using its XMP profile (which you want to do), best price / performance ratio, 2x8GB and 3200 Mhz is perfect especially for the platform etc.
Disk: Kingston A2000 500GB @ €52.99
Kingston A2000 (SA2000M8/500G) SSD NVMe PCIe M.2 2280 500GB - Kostenloser Versand ab 29€. Jetzt bei Amazon.de bestellen!
Really fast, best price / perf, NVMe so plugs into the motherboard (which does have the required slot).
PSU: Corsair TX650M @ €65.93 (warehouse)
Assuming that deal is valid, it's a high quality PSU much better than you can expect for the money... marked as acceptable which is fine for a PSU (unlike a monitor!) just make sure it has all the cables you need etc. It's also 140mm deep which is the max that will fit comfortably in the case.
Assuming all those deals are valid, add in the €77.90 for the case and that would be an awesome mini beast PC for €554.02 + graphics card (+ monitor). Honestly, one of the nice things is you could use literally any graphics card, so you could get a cheap one for now if budget doesn't stretch or deal not available etc. Can't see much else on amazon.de, but you could pick up a 1060 or something for pretty cheap on ebay, e.g.
Entdecken Sie KFA2 GeForce GTX 1060 OC 3GB Grafikkarte - 60NNH7DSL9CK in der großen Auswahl bei eBay. Kostenlose Lieferung für viele Artikel!