New detached 'one room' studio build- but which combination of shell and inetrnals?

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Hi

I am just about to talk to a local and trusted builder, and also to the planning authorities where i like in Suffolk, UK because I need to build a detached studio in my back garden where I can work every day all year round.
I have a max budget of £12,000. I am not a builder but am willing to get my hands dirty if the budget isn't enough :-)

I'd love to know what your advice would be on the following:-
(a) Shell: Should I build the shell from brick? Should I forget about building the shell from timber? Flat roof of concrete or pitched roof timber frame? Likely max building height allowed will be 3 meters.

(b) Internals: In Rod Gervais' book he details the design of a 2 leaf double wall (STC57 and STC63). Does this entire double wall sit *inside* the brick shell with an air gap between brick wall and Rod's wall? Or have I misunderstood the book and in fact the outer leaf of Rod's wall *is* the brick wall? Whats the best combination of shell and internal wall/floor/ceiling that you think I need to use in order to achieve the isolation based on above scenario/figures?

Here's some more info about what I'm aiming for.....

Studio Use: I will not be a commercial recording studio, there wont be any drumkits or loud guitar amps to worry about, and I monitor at reasonable levels. I want to build a 'single room' studio so wont have a two room set up.

Lets assume I get permission to build a new detached structure from scratch at the bottom of my garden. This would be 70 feet away from the nearest house, and my garden backs onto farmers fields on the outskirts of a small and quiet village. Max slab size would be 6mx5m

So I did some basic tests using the dB iPhone app and the handsfree kit mic (i know its not great but I had to start somewhere). I fired up my own 25yr old honda 4 stroke petrol engined lawnmower for this.

This is what I got:
1.Standing right next to the petrol lawnmower on full throttle: 90db
2.Standing at studio site, other side of a shrub/bamboo hedge approx 20 feet away from lawnmower= 66dB
3.Standing at studio site, mower engine switched off, with a combine harvester working in a field approx 75 yards away: 54dB
4. Same as (3) above but a shotgun was fired twice somewhere near the combiner as a small truck drove past the house: 61dB

I also just took a reading in my existing room, sat in the listening position and it gave:
My normal monitoring level= 88dB max
My loudest monitoring level= 94dB max
Probably not the most scientific of methods I know, but hopefully gives you an idea?

Thanks in advance for any advice you can give!
Best regards,
Al.