I would assume many of the parts are reusable. He has a Radeon 4800 series, which is not a low end card. That likely means case and PSU are decent and the drives are surely SATA which can be reused. We are talking a CPU/RAM/MB upgrade only.
I think you could do an fx-6300+MB+8GB DDR3 for 200 Euro. Assuming we don't have to buy an OS as mentioned above.
G2120 is an ivy bridge processor, which often requires a firmware update in a h61 board. So it's a bit risky to use that one with an older board. B75 is a nice cheap chipset for ivy bridge, or you can just drop to like Pentium G860 instead.
For video, I still like the additional cores of the AMD at this pricepoint.