When solar panels on the garage are facing south and the neighbors wall is directly against it the panels will only see halve
of the sunlight and produce halve of the energy.
The roof of your house seems better suited.
Inverters range between 500 and 2000 euro depending on quality and capacity.
-is possible to use at the same time both Source Mains + Solar?-
I assume you want a system that is tied to the mains grid,
The inverter syncs to the mains frequency and delivers power to the mains grid, your electricity meter
counts backwards.
If you're consuming power at the same time it gets subtracted from the produced power and you meter counts slower backwards or forwards depending on the amounts.
If you want to do some extensive calculations you good use the free sunnydesign web app from SMA
https://www.sunnydesignweb.com/