As the appearance of the garden has a modernist style with the straight lines and concrete furniture how about a Mondrian influence and add a bold strip of blue? It is a pond and with the right shade it could look really relaxed and contemporary. My daughter's suggestion is a chalky lilac!

I can't seem to figure out how to get a great picture of a tongue and groove living room that also has ceiling molding, off of one of my houzz ideabooks to show you that this style you like, can and does look good together. I picked the collaborate feature and put your user name in it to try and share this pic.

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.

As chookchook said in her recommendation about painting, put a drop cloth on the floor. Get a very safe ladder. remove the lightbulbs, and cover the outlets. Turn off the electricity to the chandelier. Then spray it with a mild cleaner (I use one I found at Target that's basically water with vinegar, until it's dripping wet. Start at the bottom and work your way up, so you don't have dirty spray dripping on the bottom of the chandelier.Work your way up and down until the dripping spray is clean. The chandelier may be gorgeous and expensive, but it seems totally out of place in the architecture of the room--more at home in a chalet than a sunny room with a window wall. I would probably try to find a way to get it removed, and sell it to someone who appreciated it better than I.