Hi Sarah,
Welcome to the forum.
You have given us an interesting project. My brief thoughts, without much detail of your requirements, as an enthusiastic ‘newbie’ gardener, would be to keep things simple to start.I suggest you select areas which are most open from shading , away from the tree canopy and ivy clad wall. Mow down or remove as much weed as possible. Cover with cardboard and apply 4″ of compost/manure. Cover these beds with opaque sheeting , such as poly tarpauline for the winter. Weight down well.
From minimal research on Vienna climate, I think your effective season for outside growing is likely shorter than here in UK. You can forward raise plants according to your tastes for planting out in Spring. Maybe the spare room would permit?
You mention regular intervals of absence. If these are in 10 day blocks, you may need to arrange caretaking in some form, if its just weekends , then you will more easily cope..
Members of this forum will always be happy to reply to questions
Good luck
Cleansweep
