My tip if you do not want to pay much for a motherboard, memory and CPU bundle, go for a "5V CPU only" VLB motherboard bundle, they often go really cheap.
I just bought a nice VLB motherboard with memory and a DX2 CPU on Ebay 10 minuts ago, the price was £25.99 = 35 euro add a video card (10-15 euro), an I/O card (5-15 euro), a floppy drive (1-5 euro) and use some old ATX case from your basement with a 5 euro adapter and you are still below 100 euro even with the shipping costs.
If you go for the most attractive parts its not hard to spend 200+ euro on a computer that does the same thing a P100 woud do even better for less than half the price. I buy both the cheap end expensive though but I really like old hardware and the challange of taking aging tech as far as possible.
New PC: i9 12900K @5GHz all cores @1.2v. MSI PRO Z690-A. 32GB DDR4 3600 CL14. 3070Ti.
Old PC: Dual Xeon X5690@4.6GHz, EVGA SR-2, 48GB DDR3R@2000MHz, Intel X25-M. GTX 980ti.
Older PC: K6-3+ 400@600MHz, PC-Chips M577, 256MB SDRAM, AWE64, Voodoo Banshee.