Found this good looking old school rockhopper comp for 100€. How doable / costly will it be to swap the fork for a rigid one?

Extremely doable. Pretty sure that bike came with a rigid fork to begin with, unless it has ‘FS’ in the name.
You need to know if it’s either 1″ or 1 1/8″ and then get a steerer longer than the headtube to account for either a threaded headset or a threadless one. (even longer.) And the headset to match. (and stem if you plan on going threadless)
That fork looks like the elastomers are shot.