There are a number of logistical issues. When you batten out the walls and fit wall board that will add depth to the walls. That means that some items cannot go back where they are at the moment. particularly the main fuseboard and the 4xsockets next to it. They will no longer fit behind the garage door.
All of the wiring will be too short as you will need extra to bring the cables through the extra depth (and sound insulation??). You may like flush boxes and fittings rather than surface boxes. Same for the ceiling.
so it may need a complete rewire and replan plus some new accessories. Not something that can be costed or even estimated on an Internet forum.
Get three quotes from electricians.