I suggest also looking at hiring one. I worked in a place that had a 60ppm duplex scanner that could (and did) run almost continuously. It cost about $50k, and looking round the net it seems that those things have dropped in price but not enough. So even paying $500 to hire the scanner plus PC it connects to for a few days might work out cheaper than buying a slower one. 36,000 pages per day assuming you stick at it for 10 hours... then you just have to OCR them (I'm not sure if that was built in, we didn't have any use for the feature if it was)
My approach is to buy an A3 scanner because I want to scan magazines, and I'll probably try to build a home-made book scanner like this guy did:
http://gizmodo.com/gadgets/periphera...ner-156334.php