If someone says that they want a 6M x 6M extension I would assume that they are the external measurements.
For the purposes of obtaining planning permission you would state the external dimensions.
Assuming 6 metres is the external measurement of the extension, you wouldn't be able to install 5 metre wide doors without additional support, as Building Regulations stipulate that the masonry piers either side of the doors must be a minimum 785mm wide.
1) Go for a concrete beam suspended floor with block infill. Quicker to install and should give a cost saving.
2) Definitely warm roof. Again, quicker to fit, no need to ventilate the roof, and keeps Building Control happy. The downside is that the overall thickness of the roof means that the...