I can definitely understand that you include what money you could have. We did that too; we had the wedding firmly planned and counted on the change in tax classes. Support from my parents, because I am still studying, was also included in the income, etc.
I do not see it as unrealistic that you could get a building permit by the end of 2020.
First, look for a plot of land, simultaneously visit model house parks to see what you like. (And what that would cost). Find out what is possible. Do not sign anything, no matter how well you are supposedly going to get out of it.
If you start putting effort into saving now, you will have quite a bit together in a few months.
We now have almost exactly 2 years after our first search for land the notary appointment, because the development of the new building area took so long. (Nature conservation statements and so on delayed a lot, plus archaeological excavations.) But in the meantime, we have already prepared enough so that we will submit the building application a week after that. Moving in will be at the end of 2020/beginning of 2021.
For total costs, I would rather expect around €450,000, depending on price increases that may still come, probably a bit more. (for land, house, ancillary costs; with own contribution)