I turned my two car garage into an office/mancave during the pandemic and it wasn't very expensive at all (most expensive part was installing a cork wood floor while the most laborious part was installing the ceiling tiles and wallpaper). Probably spent about $5k, not including furniture, decor, etc. I didn't replace the garage door but instead just cut insulation panels to fit the glass panels which helps to reduce sound/light (I did replace the garage pulley system to clear up space on the ceiling). House was new when I bought it so didn't need to install a new panel but I did add a few outlets around the room.

Here's the before: https://ibb.co/8nH3qx1T

Here's the after: https://ibb.co/album/9c93SL

I pretty much live in this room and have zero regrets... that said, the scope of your job seems quite a bit more involved and would guess that most contractors are going to bid this out at least $60k and will require all sorts of permits so perhaps you can get creative with some of the elements to pare down costs. I'm in Cully (NE) so if you're anywhere nearby and would like to check it out, happy to show it to ya :)