Agree with the others. Make a business plan - doesn't have to be complex, but figure out how you're going to market yourselves and to WHOM. Who is your target? Put an ad on Craig's, use the school paper or bulletin boards.. talk to other people and mainly do your research. Read specs on the egg for pre-builts and newspaper ads.
It is VERY hard to second guess what your "clients" will want. Decide if you're going to go lowball, mid-range on the components. But.. buy quality so you don't get returns. Watch the combo deals. Cheaping out will cause returns and that's BAD news. Quality Power supplies are #1 on the list. You may only need 300W but get good ones. Read Tom's on PSU's.
Building shouldn't take long so charge $25+ flat rate? add another flat rate for the OS. Remember you will probably get calls for support, too. You guys know how to trouble shoot? Don't over spend. Don't over spend. Don't over spend.
It's ok to provide a parts list to pick from. But, yeah, you're probably better off going low end.. uh budget. READ the guides on here and on Anand and elsewhere. I'd trust you guys more than BB for sure! OFFER to install anything and everything, too. A potential good market might be the E350's and some E450s will show up soon. ITX. Can do that with wifi kbd/mouse for under $300. Not much power but they'll do for surfing and school work. About the same for AMD 880G uATX. (no OS) IGP are just about good enough. Remember - you guys can BUILD what you can't BUY off the shelf so it's higher value but that doesn't mean much when the customers only want to pay bargain basement prices.
Do your research. Read reviews. It might work, it might not but if you don't try, you'll never know. Maybe you could attach yourselves to mom and pop shops and do what they might not want to do. Maybe they'll let you place a rig or two.. Good luck but don't get disappointed if it doesn't take off.