You're biting off more than you can chew here. Photographing the artwork for reproduction means you're going to have to learn about and invest in proper lighting, and you'll be limited in resolution quite significantly compared to a scanner unless you also learn how to do multiple row & column stitching, which is only practical with digital gear. A flatbed scanner smaller than the artwork is a really bad idea; you will probably damage the artwork as you move it around on the scanner, plus you'd have to disassemble it partially to remove the lid, which might cause malfunctions, and would void the warranty. A scanning back for a medium or large format camera (like a Betterlight) would work well, but you'd still have to invest substantially in lighting, the back, the camera, lenses, and the computer hardware to handle large files. Any route you go, you're looking at a minimum of $5000-10000 in equipment to do the job professionally.
Given that, hiring a competent professional photographer to digitize the art for you is not going to be cheap; expect an hourly rate of at least $100. IMO you should look elsewhere for money-making ideas. Given the budget and equipment you're looking at, and your experience level, at best you'd be able to do a half-assed job, which isn't going to make your clients very happy.