There are lots of good suggestions here, and since you're in the Netherlands, you ought to be able to get whatever you decide you want. As for how to decide, here are some questions:
1. How mechanically inclined are you, and do you want to do all your own mechanical work? If you're willing to do your own DIY conversions, you could get exactly what you want fairly cheaply -- but it'd mean that the bike might be harder to get fixed in a hurry at some small-town bike shop in the middle of nowhere.
2. How much do you want to spend on it? Everybody's suggesting 650B conversions, but by the time you buy new wheels, new tires, and (probably) new brakes, that cheap old bike won't be so cheap anymore.
3. What kinds of "gravel shortcuts," really, are we talking about? In my experience, on fairly flat ground, really fat tires aren't necessary -- unless you'll be riding in soft sand (say, near the beach). If you're talking about gravel country roads and no mountains, 35mm ought to be fine.
4. Taking it all together, my recommendation would be a late-'80s touring bike -- something like this:
https://www.bikeforums.net/classic-v...ta-1000-a.html Defining features: cantilever brakes, triple chainwheel. Most will take 35mm tires without fenders.