Converting brick built garage

Zulu

We recently moved into a new build with a separate brick built garage in the back garden. I'm planning on converting this into a room. Myself. I'd greatly appreciate any help advice and guidance you can offer/provide.

The garage building itself is a "semi-d" under a pitched roof. The neighbors garage is connect to ours and it looks like a small house with a standard roof - it you catch my meaning.

The floor is concrete laid over a moisture barrier (I can see the plastic when the garage door is open). There's a plaster inside ceiling on the inside, and while there is no access to the "attic", the builder told me the internal brick wall dividing my garage to my neighbors runs to the apex of the roof (separating the two garages).

I'll be using it as a workshop/home brewery for the next few years, and following that, a playroom for older kids as they need their own space (in say 5-8 years). My plan (due to finances) is to do this in stages as i save money.

So, the first thing I'm thinking of doing is putting in a floor, and lining the walls.
To do this, I've a bucket load of pallet boards that have been power hosed clean and I was toying with the idea of using them to clad the partitioning wall, and to use as a wooden floor.

I was going to spend some money and line the exterior walls with insulated plaster board sheets.

I appreciate that until I fill in the garage door, it'll be cold and super drafty, but I'm ok with that for the moment.

So to clad the partition wall, I was going to screw in small batons running vertically (from ceiling to floor), insulate between the batons, and then hammer away with the pallet boards (horizontally). Job done!
First question: Seems simple enough, am I missing anything?

Then for the floor, I was going to get some floor insulation, lay that down, and then float the pallet board floor.
Second question: As it's not "tongue & groove" am I going to be getting in way over my head here? Bearing in mind, I'm not too concerned about the "look" of the floor and it's going to be rustic and battered looking anyway, so maybe I can just screw the bejaysus out of it to the concrete?

For the exterior walls I understand that I can just pin the insulated plaster board to the wall easily enough, and then either try to skim a finish over it myself, or I've a friend who can help me with that.

Finally, is there an order I need to consider when doing the above?

Thanks for taking the time to read...