How large is your market? Are you just going to be divvying it up with your former employer, or are do they have only a fraction of a much larger market? If you're in a small town, the pickings can get very slim.
I hope you plan on aiming higher than just computer repair. The real money will be in small businesses, computer and network setup and maintenance. And, hopefully, you can do something beyond just hardware, which is a commodity.
As to the "mobile" thing ... IMO, it's not worth much, if anything at all. That doesn't mean you need a storefront, but you also don't need to fit out a van and carry every conceivable spare part around with you. I know people who run similar businesses from their homes. If they need to, they bring a computer back to the house, fix it on the kitchen table, return it that day or the next. As for on-site repairs, you could carry 99% of what you need in bicycle messenger bag.