You may find that having an old inefficient boiler will lower the property price within it's bracket rather than a new efficient boiler will increase it. Anyone purchasing will take into consideration that a new boiler will be needed therefore drop the price to reflect that. Adding a modern condensing boiler will certainly maximise value but not increase it per sé.
If high HW usage is a requirement and further extension of the house is possible/probable with the obvious increase in HW demand, then the better alternative, if the mains cold water can support it, would be to provide high capacity, high recovery stored hot water, coupled with a correctly sized efficient heating system. That can also be a very positive selling point, again maximising the property value towards the upper part of the price bracket.
If high HW usage is a requirement and further extension of the house is possible/probable with the obvious increase in HW demand, then the better alternative, if the mains cold water can support it, would be to provide high capacity, high recovery stored hot water, coupled with a correctly sized efficient heating system. That can also be a very positive selling point, again maximising the property value towards the upper part of the price bracket.