You have far more experiance than I do, and I don't want to be too negative, but.........
I think the price should reflect how underpowered it will be with the 90HP on.
When I had an old 150HP on my boat, which is basically the same hull, it would top out at @29knots, partly because they are heavy, but sturdy, old boats, and the engine was probably slightly down on power.
Its plenty lively now with the 200Hp on
With 4 adults on board and a full tank of gas it may struggle up onto the plane with a 90HP.
As it is with a trailer requiring work etc I'd say @£4.5K max.
I honestly think it would be worth more being split up, as the Honda 90 would be snapped up for a smaller boat. Although as I said, you would know more than I.