Hi
We are building a house, but with 2 residential units and a large garage, somewhat like you are planning.
Swedish slab, timber frame and shed roof except for parts of the groundwork, 14 days with a carpenter and the installation of the windows done by ourselves. I have already written a bit about the costs -- see [Pultdachhaus die Zweite]
I will add pictures of the current status tomorrow.
I also think your cost estimate is a bit too low. With €100k equity, it is certainly not a problem to finance another €20k.
For the bank, it was no obstacle with the last house and also with this one that we do some work ourselves.
Olli