I think we've been spoiled by VVD, plus Joe, Ibu and Joel (when fit and at their peak). If we want to play with a high line, holding halfway and playing the game in the opposition half, then the centre backs need to be fast and able to cover large spaces, plus dominant aerially. Guehi has not played in teams with that style either for Palace or England. He has clearly played well in teams that are defensively set up, however when i've watched the occasional england game i've seen him make mistakes on the ball and not look particularly fast. Plus the obvious weakness in the air, which would be more exposed here as one of the ways teams try to get at us
The problem we have is that its a very high bar for what we need in a centre back, which is possibly why we haven't been buying them as we are essentially looking for a unicorn. To answer the question, Bastoni would clearly be the level we would want. I actually think Quansah in 2 or 3 years if he develops well could be the right profile. I don't watch enough other football now to be able to say who else is out there