A new semi-passive 850-watt fully modular PSU is around EUR 130, the Noctua fan around EUR 30.
I guess if you know electronics and how to safely handle the PSU internals, the risk of injury is low, but I personally would not risk it for EUR 100.
Also, if the only problem was the noisy fan, I guess selling it used would have returned most of the investment, leaving him with like EUR 50 in added cost. Compared to the price of a modern gaming PC, that's nothing (also avoiding not risking your life).
I guess if you know electronics and how to safely handle the PSU internals, the risk of injury is low, but I personally would not risk it for EUR 100.
Also, if the only problem was the noisy fan, I guess selling it used would have returned most of the investment, leaving him with like EUR 50 in added cost. Compared to the price of a modern gaming PC, that's nothing (also avoiding not risking your life).