I own a cx7800 epson all in one. Which means I have a scanner, copier, printer all in one. Why buy a stand alone scanner. Depending on the price your quality of resolution increases with the printer price. I have owned stand alone scanners and they do what my all in one does and I don't need another piece of equipment.
I have a printer. I need a scanner and dont want them to be together. If the printer goes down, I don't want to be with out the scanner. If the scanner goes down, I still want to print while it gets fixed.
The epson perfection 3170 is a good one. The higher the resolution your scanner the better. The higer you go the more money. Also depending on the size of the scan. 8.5 X 11 or legal or bigger.
I would say 8.5 X 11 should work just fine. (its what I have) If you need anything bigger scanned take to a digital prepress company and have a good drum scan made.