Best way to advertise that type of business is reputation.
As for the local IT business stuff, I can comment on that. A good majority of business professionals I work with hate their IT guy/company. The way that we got our recent one was his reputation with several other retailers that we work with. He is much better than the guys we previously had too.
But, other than that. Local business journals, business sections in newspapers, radio ads on stations with a listener demographic you need, TV on business channels, have a good clean simple & professional looking webpage.
Honestly, I don't see how there is money in custom systems. Everyone of the local guys are either a whole hell of a lot more money than Dell, HP, Toshiba, Acer, etc etc or they build junk systems for less.
Honestly, for our company I would NEVER buy another custom 'local' built system because we have had nothing but problems with them. Right now we have all Dell and I can't complain, I don't remember the last time I've had to replace something in any of the computers.
To start anew be prepared to have some serious advertising costs to start. Reputation is important important important. If you can stay a float long enough to get a good/great reputation and you won't need to spend so much on advertising.
Anyway, I wish you luck in a very cost cut low cost business.