Just my own opinion, but, If I were looking for a new boat (and I`m not , thats for sure) I would probably look now toward an outboard instead of the more traditional Inboard.
The inboard diesel was always my "must have" option due to reliability and more than anything the price of red diesel. The factor of power was always a trade off because of this. But now, with red diesel at virtually £1.00 per litre (on 60/40 duty) and the obvious view that the 60/40 will die away very shortly, then the efficiency of new 4 stroke petrol outboards and their power to weight ratio against diesels now place them probably in front of inboard diesels.
As an added point, if you do decide to keep your favoured boat for a number of years it is not possible to upgrade and "re-engine" an inboard deisel very easily, simply technology moves on so fast that new engines do not fit old fittings, however to re-engine an outboard is fairly straight forward.
Food for thought .......... all due to duty payments on red diesel, still, we shouldn't complain I`m sure the duty they collect is used to better the waterways ........................
norm