I am about to trade in my old iMac OSX Tiger for the latest iMac. Will my HP Photosmart 7350 Printer and my Canoscan 8400F scanner be compatible?

check the manfacturers websites to see if the they have Lion drivers.

Start with http://support.apple.com/kb/HT4670

There is no Lion driver for your CanoScan 8400F.

I regularly use my 8400F scanner with Lion. It works with both canon's software that came with the scanner (probably updated, but I do not remember those kinds of things) and also with vuescan. Good luck.