I really don't see an issue with the rooflines. Looks like the angles are fairly similar and the clean lines of both structures should be easy to coordinate, especially in your open, rural setting. Since you've found a stunning deal, I would say, go for it! I definitely would want to find an architect ASAP and by all means, before you pour the slab, make sure you have a plan to coordinate.

This is a similar home. Does the architect's response help you? "Yes the soffit is painted ( Resene Bokara ) to make it similar to the cladding. We did it this way in order to emphasis the sculptural form of the house - if it was white it would have broken up the form somewhat. I think it depends on what you want to achieve, if the line of the joinery is at ceiling level then it is good to have the ceiling and the soffit the same colour in terms of how you experience the interior space. If the exterior look is more important then the dark soffit would probably work better."

Led lighting running on both sides of each of the beams - you can buy it in strips. I would go for a bright white but you can even get it in colours and or changing colours. I would also do the same along the floor under the cabinets and perhaps between the upper cabinets and bench top.

The heart says keep it , but if you have to move it , the ducting will be a problem , no matter how far you move it . even if you can add bends and use the existing exit ( normally the ceiling then roof ) some ducting will be new and shiny , some 30 odd years old . Basically , if its $200 , I'd do it , but much more than that , probably not .