Without wishing to rain on your parade,
Don't do it, you will be disappointed with what you get.
Brushed stainless or ally, despite looking fantastic, is very hard to achieve,
The brushing needs to be uniform, and is very directional, Once brushed, Ally will require lacquering, otherwise it will oxidise and dull.
Stainless, will show water spots everytime it rains, and drys.
Also if you have anything in contact with the tank, Knees, jacket etc, all will eventually brush a new pattern in the tank
You need a strict regime for washing the bike, as the tank will be susceptible to surface scratches from you cloths etc.
Any damage, stone chips etc, will not be able to be filled and polished / brushed out.