Alright,
personally, I have no experience with megaport. One of my go-to shops in the Netherlands is Alternate, I know they deliver in Germany as well and have a German online storefront with PC configurator. They also have a reasonably priced option to build a system for you. The only thing left to insert yourself after delivery in that case is the graphics card. Likely done to prevent shipping damage.
Considering that you are not in a hurry and have a high budget. I would suggest the Ryzen 7900X CPU. The best compatible RAM for this is of the DDR5-6000 type. In which for best performance limiting yourself to 2 bars is recommended. With that, there are kits available which bring you to 64GB of RAM which is far sufficient for any game these days and will be sufficient for quite a number of years to come.
What you’ll furthermore notice with RAM is CL. This stands for CAS latency. The higher the number, the greater the latency. Hence, perhaps a bit counterintuitively, you’ll want a low CL for better performance. With that in mind, DDR5-6000 64GB 30 CL kits are best compatible available on the market currently that I am aware of, from G.SKILL.
GPU wise I would wait for AMD’s announcement, to see how price competitive their lineup will be. The RTX 4080 12GB/16GB variants in my opinion are having a too high RSP in my opinion. A 3080Ti could be an alternative, in case AMD isn’t price competitive and the 3080Ti lineup doesn’t vanish from the market completely. This obviously being a bit of a gamble between waiting and not.
The last truly important piece of hardware these days is the power supply. Depending on the combination of GPU and CPU ultimately picked on, take one which is compatible, and don’t go for some cheap off-brand product. Consistent power delivery, as well as sufficient power delivery, is becoming more and more important with high-end hardware under stress quickly demanding 500+ Watt these days.
For SSD’s my personal go-to brand has been Samsung for years. Thus far never failed me, not in regular SSD’s, not in M2 SSD’s.
Motherboard wise, don’t go overboard. Anything in the 200 - 300 Eur range from one of the top-tier brands is by far sufficient, as long as it has the right socket. The 300+ range offers no extra noticeable performance. And even 200 - 300 Eur boards, offer little to no noticeable performance improvement over a 150 - 200 Eur range, but just overall handy connectivity features. So basically, look at what you need, that being e.g., USB connectivity, WiFi, Gbit LAN connection, etc.
And lastly, myself I’m a major fan of large tower cases. Specifically, for the reason you mention. New parts may not fit in mid towers or worse, small cases. I tend to last roughly 10 years with a CPU these days, with a GPU upgrade cycle of roughly every 3 years. It’s nice to have the space for this in the case. Large cases also add the advantage of much more fan space, which can be run at lower RPM and hence less noise.