1200mm or 4 feet would be ideal.

We are a little biased, but wood floors are versatile, durable, clean, and timeless. Whereas carpet will eventually need to be replaced, high quality wood floors can last forever. You can install radiant heat under the wood floors if you are worried about them being too cold during certain times of year, and you can easily install area rugs over the flooring. Wood flooring is also more versatile than it has ever been so you can create any look. As an added bonus you can easily sweep and mop your wood floors, whereas with Carpet you don't always know what is lying below the surface. Good luck!

You are so right white is never out of fashion. I would set your budget and then look at your options. If you plan to stay in the house for many years then spend to the top of your budget (if that is going to give you a life time of pleasure), if you plan on being at the home for only a short time / few years then I would go mid range so it looks lovely, will have good resale value but you can invest your well earned money to go towards something else as well. Obviously Marble / Granite is the top of the range but there are so many cheaper and very attractive options these days that look just as good.

The materials will need to meet the requirements for the hearth for the woodburner you choose. They need to project a set distance forward of the firebox opening as well, unless you have a concrete floor or other fireproof material. The structure needed will also depend on the weight of the woodburner you choose. It will need to hold that weight and hold it against sideways loads for earthquake safety. If you want a thin looking one, you may need an engineer to design it.