We conduct All India Mock Test Series quite frequently and it was a year back we decided to automate the answer evaluation process. We bought HP 7450C. That was after we considered some other more costly scanners, and went mostly after price, cheapest, that is. Though HP is relatively good at lower end of products, HP 7450 is a disappointment. Right from the start, it has been a disappointment. With people who came in to install it taking a whole day to install, having no idea of what to do with this "High End Scanner". Had problems with the first lot of scanning itself. Unlike printers, where your page orients itself straight when it goes in the printer, this scanner has no such process, and the scans are mostly tilted. ADF jam is the most common problem. After an astonishingly few scans, the roller becomes dirty and the sensor at the paper insertion point does not detect paper when it is inserted. There is no method to clean it either. HP service center is usually overcrowded, with uncaring staff, who cant help it if so many people end up buying HP products. HP does not work at component level. Which means that if, figuratively, a screw in ADF is lose, they will change the whole ADF instead of tightening it. So buying an extended warranty is always better. We got a pleasant shock when we learnt one month after buying the product that the warranty could not be extended since the product was declared obsolete, and was no more under production. Luckily, our product was in warranty period, so we cajoled them into it.
Our scanner currently lies in Bellari, where HP service center "accidentally" sent it to, and we have been given a standby scanner to work upon, which came with scratches on flatbed glass, and a call from the manager with "kindly adjust for some days". And today the service center has told me that the scanner given to us is a new replacement, and not a standby.
When I consulted a Singapore based OMR software company, they had suggested fijutsu. Whichever you buy, avoid HP like plague.