Windows XP hardware compatibility spans nearly 20 years. Yet, I've found that Windows XP and a lot of 32-bit software will take as much compute performance as you can throw at it. If you aren't nostalgic for particular hardware of the era, I recommend that you set a budget for each component, and then buy the highest performance component (yet still compatible with XP) within your budget. I recommend you stay with major components introduced before 2015 to maintain compatibility with XP. Anything later gets into "unsupported" territory which I don't suggest to anyone who doesn't like to really tinker with hardware and software.
I don't know the purchasing power of the Euro in your market, but I'll base a recommendation on US Dollars which should be a relatively close estimate. Some parts you will only get used these days for reasonable prices, but other parts you can get new for a little extra life. I assume you already have accessories (monitor, keyboard, mouse, etc.), so this is the budget and suggested part that I would get if I was spending my money:
Motherboard - $50 - Z97, H97, Z87, or H87 Motherboard (Used)
Processor - $100 - i7-4790k (Used)
RAM - $15 - 8GB DDR3-1600 (New or Used)
GPU - $50 - GTX 960
Storage - $50 - 1TB SATA SSD (New)
Power Supply - $80 - 750W ATX (New)
Case - $50 - ATX or MATX Case (New or Used)
Optional:
Optical Drive - $20 - SATA DVD +/- RW Drive (New or Used)
Sound - $30 - PCIE X-Fi Titanium
Networking - $15 - PCIE Realtek RTL8111 Network Card (New)
Storage 2 - $50 - SATA 2TB HDD (New)
This should all fit within your budget. You can do a little better on individual components, but this set will give you an awesome machine that will play the original Crysis high settings at more than 60 FPS at 1080p. It will even handle XP-compatible games released in the 2010's without any trouble.
The optional components are completely personal preference. I still have optical disks in my collection, so all of my builds have at least one optical drive (most have 2). Integrated sound on the motherboard will likely work just fine with Windows XP. If you want something a little extra, an X-Fi will give you positional sound in a lot of games from the late-90's to the mid-00's. At the same time, some integrated network cards will not work with XP (too new). If you need to network, a cheap Realtek network card will do the job. Finally, I like to use a spinning disk for extra storage (drivers, software installers, etc.), but not as the boot/application drive. I also assign the virtual memory to the spinning disk for a little better I/O performance.
