2. The 12V option does nothing (in and of itself to the sound). However the 12V tubes in some cases can run quieter and such (are are cheaper generally than their 6V counterparts. As far as using 6 or 12V tuybes goes, it depends how he does the amp. Just make sure you tell him what you want (6/12V gain and 6/12V outputs; 6/12V gain and 6V outputs, etc....).
3. The 6BX7s are a more neutral/clean sounding tube than the BL7s. They are both octal and can be run hard, but which one to use just depends on preferences and synergy.
4. The regular MPX3 transformer is fine for this/