Good question and I think it's a matter of taste. But, I've found that people who purchased "bar height" kitchen tables did not like the everyday "bar" feel of sitting at a table. So, in that regard, I would think a "normal" height eating space would be more practical than bar height at the end of your island. But, if you have a "formal" eating area where you can dine at a standard height table and will use daily, that would be a consideration.

I'd keep the island the same height- bench height is taller than a table...I think around 90cm

I guess it depends on how much walking space you want. In this current configuration you have quite a lot of space for moving around, but do you want that? I would personally go for an L shape to maximise bench space with one section at a different height for seating. Alternatively, you could just have a standard L shape with seating on one side. Either way it's going to be quite a nicely sized kitchen.

Any range hood extractor fan would be suitable. It depends on the aesthetics of your kitchen, and your budget as to which style would suit the best. Have you thought about a down draft extractor fan?