.......just sit back, relax and hire someone else to do it.

You do realise you are changing use from garage to habitable, which will require building regs.
All proposed works and materials will need to comply with regs.

In reply to your edited question, if you go for the stud frame, sit it on a course of bricks, then dpc, keep as much cavity as you can, when you fit celotex, fit some membrane, tape all joins then board.

Presumably you have no approved plans to follow and as such,cannot call out the building inspector for advice? You can find all the information you need on the internet.There are various ways of doing things and you will find what best suits your particular situation.It will explain damp proofing etc too and its vital you get this right.Brace yourself for some sarky comments too as there are a few people on here that think that they are gods gift to building and may comment on the fact that you are in the game but seek advice.If only we were REAL builders like them lol.

You would think that a builder with 15 years experience,over 30 if you count his workers would get their heads together and work this one out for themselves wouldn't you? :/

Hello.
You don't mention if the garage is single skin or a double with cavity.
If its a cavity wall why not use 50mm insulated plasterboards dot and dabbed
And over insulate the ceiling to meet u values.
If you use 3x2 stud work you will need a vapor barrier on the back of the stud work.
If you have had the inspector out just follow his advise so it complys with regs.
Regards
Chris