What is actually in the other part of the barn that it cannot be torn down? Did that go past me???
Basically: turning an old barn into a cool dwelling is possible, very cool results can come out of it. But, now the big BUT: it costs. A good architect (and from what I have seen of your architect so far, unfortunately this raises the suspicion that he does not belong to this category), a lot of financial leeway for structural gimmicks, unexpected "discoveries" (I also feel uneasy reading about the floor situation), pipes (whether water or electricity), and also going beyond the size of imagined limits. For example, for me now, the idea stands to think about opening up the attic. Whether that is in the form of a gallery or simply a (partially used) upper floor is currently irrelevant. But, there we already have the first hurdle: fire protection, which becomes too expensive. Already at that statement, I would keep my fingers off such a project.
Whoever wants to do something like that and aim for a good and nice result (which possibly also has resale value someday) has to have money and be willing to invest it there. Such a tight budget on such a project would give me cold sweat on my forehead.
Then it is a metal roof, if I remember correctly. How loud is it when rain is pounding on it?
In principle, I do not understand why one would so desperately want to preserve and convert a barn from the 70s (with accordingly lacking charm). I could understand that with a really old building that looks so cute from the outside that one is willing to tackle and finance the internal challenges, but what I saw in the pictures would first prompt me, even without renovation measures, with the idea "tear it down immediately". The building is ugly as sin. Why should it be preserved?
Because of the new roof on one half? Because of the photovoltaic system??? Seriously???
Hence my question: what is housed in the other half?
Even if your financial leeway is limited, do you really want to pour so much money into a building that will always remain ugly and demand disproportionate compromises without delivering any value in return?
I agree with 11ant, the grant of €50,000 sounds like a lot and good, but with the requirements you will have to put in far more to get a somewhat acceptable result.
As I said, with an old, historic building, I would understand the effort, but like this?
Flatten the barn, find a solution for the other half, build a single-family house that meets modern requirements without contortions, possibly transfer the photovoltaic system to one of the new buildings, and enjoy the beautiful plot with a house that fits you.