I am going to go through the campaign on Torment 1, and will write down my gold count when I start, only using gold I earn on Necromancer.
lvl70 T6 grants the same exp as lvl1 T6 for low level characters, so you should open at lvl1 if the booster doesnt have much gear to carry you fast.
I'm pretty sure it's quite opposite - in season i usually boost on lvl 70 and it seems way faster then lvl 1 rifts.
Anyway i dont get why people want to boost new class to 70 especially when season is 4 weeks away, why not enjoy campaign in NS and discover necromancer ? (this is what i plan to do)
I do not understand why people want to rush the Necro to 70 myself either, to me it shows why so many people complain about the content, because they flat out do not remember how to even enjoy it, getting so fixated on "end game" for no real reason at all.