Several years ago I started to do the same thing. I had 7000 slides and some were starting to fade etc. I was told to look for the scanner with the highest D Max rating. I was told that a D Max below 4 would be "iffy". I found that if the advertised scanner did not have a D Max of at three it wouldn't show as a rating. I went through three different brands of flatbed with light top scanners and the best one I could find at the time was the Epson Perfection 1650 with a D Max of 3.5. I have completed the task and if I was to do it over again I would buy, beg or borrow a dedicated slide scanner. They have a D Max of 10 or higher.
http://www.kenrockwell.com/tech/scantek.htm
After glancing at this article you can disregard most of what I said regarding D Max. (maybe) The workshop I attended did stress the importance of it however.