New construction and financing - now or in 2 years or never?

I would buy the property, provided the location and surroundings are suitable. Finance the property from equity (125k + incidental costs 10k). From the spending side, that seems somehow too low to me, without wanting to generally pin it on a specific item - my assessment.

How the 90k € mentioned here is supposed to be saved in 2 years, I cannot understand.

House construction approx. 150 m² = 450k = for this a loan, we are somewhere around a good 2k monthly rate.

Everything else like garden, additional construction costs, etc. should be financed from equity (which by then will have been slightly increased again).
Garage: for what? It will just be filled with useless stuff anyway, your company car can also stand outside on a paved spot or under a carport which can be built later with some craftsmanship for a fraction of the cost.
Maybe your wife will also work more hours per week again. Some employers might not be so happy with so few weekly hours = consequently more income.

I think all in all the project should be quite feasible.

I cannot understand how the 90k € mentioned here are supposed to be saved in 2 years.

Income and expense totals:
Total income 7,300 € without special payment (approx. 10,000 € net per year)
Total expenses 3,615 €
Balance 3,685 €

24*3685 = 88,440 euros plus 2x special payments of 10,000 euros each