Hello Fusili,
I can't tell you whether the house price is realistic, architects can assess that better, but I do have my problems with the fact that the land costs almost twice as much as the house. On a plot of land that costs me 600K€, I would at least build a house that also costs 600K€ - the question of whether I can manage with 350K€ would then be irrelevant. Although with 400m² you quickly reach technical limits or build very luxuriously - which probably doesn't fit the surroundings and cannot be reactivated when reselling.
From the perspective of a self-employed person, I would initially raise a different idea/question, because basically you want to invest about 1 million € in improving your living situation. My question would then be whether you could relocate your company locally to improve the overall situation? If you can work just as well somewhere else, but the house including the plot only costs half at a reasonable distance, then 500K€ would remain as an investment in the company. Alternatively possible, although there are also good reasons not to do this: company and living in one building, then in a mixed-use area or similar.
If there is no local alternative (because of friends and/or simply because you like it in the south of M), then you will probably do (or can do) it. For 350K€ you can also get a house in M, in other regions and depending on the standard and size, prices start at 200K€, but in your situation I would also only go in with the premise of 350K€ - if in the end it becomes 400K€ or even 450K€, that would still be "underdressed" compared to the land value.
In your region - and now some people will jump up and down that I write this - you can plan the financing including the sale at retirement in such a way that paying off the loan is not the goal. This means nothing other than selling the house worth about 1 million and an 800K€ loan with e.g. 500K€ remaining loan after 20 years. You receive 1 million, pay 500K€ to the bank, and have 500K€ cash in your pocket to reorient yourself.
Or in other words: for 500K€ you only pay the interest on the capital to the bank, without repaying. Sounds harsh, but has an advantage: your monthly burden decreases (the term increases significantly, but that doesn't matter to you), which in turn means you can still invest another 50 or 100K€ in the house, i.e. the living quality, and still manage.
Think this over and discuss it with the banks. When you retire, you also have from your company the tax-advantaged business closure gain (or similar, I can't recall the exact term right now, can only be done once in a lifetime afaik, talk to your tax advisor) - depending on what values are dormant with you, that can also be taken into account.
Best regards
Dirk Grafe