can brownstone ground floor be used as a business

you need the appropriate c/o and it's true that zoning laws generally depend on where you are in the block. check before you buy.

theres probably credence to that 100ft rule
I can think of a handful of businesses in browstone basements or 1st floors and they all do happen to be very close to an avenue

It's regulated by zoning. Any residential area in which there is a commercial overlay you can have ground floor retail otherwise you would need a variance (have fun w/ that). 100ft within an avenue is very possible as the commercial overlay usually goes back 100 feet, although there is some variance there.