Try getting a
Mustek A3 ScanExpress scanner, fits
11¾ inches x 17 inches, great for artists.
I got one brand new about 3 years ago for $220 Canadian I think. (I'm in Canada. $220 Can was about $170 American at the time. Not like now where $1 Can is about the same as $1 Amer. For a while, the Canadian money was actually worth a few cents more!)
Big surface, good scans at an affordable price. I think it's been a great product because of the large scans I make.
I hear there may be a problem with out-dated drivers though; I wouldn't know because my computers a few years old, and the scanner has always worked fine. Works good with Photoshop.
Go to ebay.com, type in Mustek A3, and you'll see lots of them there.
I don't think there's any other large format scanners that are affordable like the Mustek A3.
It's been ideal for me, and it was a great investment.
http://search.ebay.com/search/search...+A3&category0=
RE shading: After scanning, you should always adjust the scan in Photoshop or a similar program. The raw, untouched scan never looks just like the original image, but if you adjust usually the Levels, Color Balance, or the Brightness/Contrast, you can make the image look closer to how the original image appears.