It would have to be a computer heavy region as the normal crowd would not understand or really see a reason to mod their computer. you could throw in console mods and maybe even some computer build and repairs as i could see most of your income being from builds and repairs. while with console modding thrown into the mix it would increase your crowd and people willing to pay because of the amount of people who like to mod your console.
Modding business idea is great. There are 2 or 3 online. But business is slim at best. You can start out as internet based, maybe have a store set up near a college. Then scower all forum sites for business. With the internet business its tuff because you add shipping on everything. Your best bet is to do this type of business off the books, word to mouth on the forums.