You only get absolute certainty on the topic of bonus from the bank. Get 2-3 appointments with different banks.
If you want to build in 2-3 years, the best idea would be the following:
Buy the land now without financing. Since you will most likely be given a mandatory building period of 3 years anyway, you have the flexibility you need. Take your equity + a family loan (if possible) and buy the land. Alternatively, a variable loan without fixed interest. Although you will lose about €2000 when transferring the land charge. You could save that if you avoid a variable loan.
This way you have time to save more and calculate your costs properly. When financing the house, your land then also counts as your equity.
Mandatory building period means: The shell must be standing by a certain date x, you don't have to have moved in yet.
This is not empty talk... because we are doing it the same way. Bought land 02/17. €55k financed variably. Paid off 12/17 and signed the house financing the same month. Building starts in 3 months.