Advice please, Log cabin, best insulation mineral wool, celotex, wood protection and varnish

Apologies as I have repeated this, I think I put it in the wrong forum previously.

Hi, am building a double skinned log cabin with cavity.
1. I was wondering on best advice for insulation of cavity walls: celotex, mineral wool, sheep wool or any other option.

2. I am looking to keep the original wood colour so was wondering on best advice for wood treatment rot, insect etc product and clear stain.

I have been recommended to use Sadolin, are there better products than others?

thanks
david