Hi Wilhelm,
if you don’t feel like fixing this issue (I mean if you’re not totaly sure of how to do it when looking at it), then my advice would be to go find another boat :)
This is no easy fix, and can be very dangerous.
kersau is facing such problems right now. I met him and I think he knows what he’s doing, and it doesn’t look like something easy for him either.
The keel is fixed to the hull with the bolts you showed, in a recessed area. There is also a seal that can be redone probably with sikaflex (ask an expert for the best for this use). To fix the problem, you have to unmount everything, and redo it properly. It’s best to have a crane to do it properly, so it’s added costs. If bolts are rusted, they will probably snap, so you’ll have to drill them, redo the thread… this is all hard times. It can be fixed but it takes time, money and experience. 
The color of the seal between the keel and the hull, is to me an absolute red flag. I wouldn’t buy this boat, for sure (I hope it’s still a buying project for you) 
.