Converting garage into man cave - advice needed

Hi all,

Looking at converting my detached garage into a man cave. Looking for advice about how to put up walls, insulation, the ceiling at what to do with the door to weatherproof it. Looking at using metal stud walls and plasterboard. Will I be okay to insulate between existing garage wall and new plasterboard? Will ventilation be required?

How much would this cost if I got someone else in to do it

Attached is a link to various garage photos

https://imgur.com/a/kybOlFF


Mike