Awesome, thanks a lot!
I was aware of Golfbidder. Actually I found some pretty good deals there (or at least I do believe so), like a Callaway Steelhead XR set for 480€, Ping G set for 515 €, or Titleist 716 AP1 set for 390 €. All at least in decent/appropriate condition, and ranked on various websites for being amongst the best SGI sets. Obviously above my budget, but compared to beginner set not that much more. They also offer a 7 days try-and-return policy for used clubs.
The only thing I don't know is whether the shafts are 0.75-1.00 inch longer as the shop clerk told me I would need. Do you know how that works on Golfbidder? I guess I'd have to ask and maybe replace the shafts (which obviously would mean extra cost and make it not a good deal anymore).
My hometown actually isn't too far from the Netherlands, so I guess I could drive there some time to check out Jumbogolf.
I did some research on beginner sets as well. Recommended beginner sets are, for example, the Callaway Strata set, the Tour Edge Reaction 3 set, and the Wilson Golf Men's 2017 Ultra set. Didn't find any of these in the shop here.
The reason I am leaning towards an SGI set (rather than a beginner set) is because of the marketing that SGI irons will be much easier and fun to play for a beginner because of the forgiveness. Is that really just marketing, or could they actually be much more fun to play as a beginner? Guess I have to try and compare...