I keep looking at KHs. I don't think I would be brave enough for a 750 (they are animals so I hear), the 400 is the sweet spot for me.
When I was 17 I would have killed for a RD250LC (I was a 125cc generation so couldn't have a 250). Now many years later I prefer the KHs, although I would move planets for a decent RD400E in yellow and black.
I keep looking for GPX750s aswell. Rare as they were used and abused, only a short production run (as didn't sell very well). They are as cheap as chips if you can find one. I never owned one but I remember a feature in 'BIKE' magazine when they first came out in 1986. At the time I was more into cars and didn't have a full bike licence, but I have never forgotten them.