Re: the scanner - yes, it's an ISA card. I had that scanner, and I no longer use it for the same reason. I believe, however, that the card is just a really old SCSI card. So if you get a modern equivalent, I assume it would work (but don't quote me on it). I'm pretty sure that Belkin make a USB SCSI adapter, but I have no idea if that would work. I don't know if the ISA card is proprietary card (which, considering HP, it could be).
*edit* looks like the Belkin product has long since been discontinued, but there are others out there, like this one. Can anyone confirm whether or not that would work?

Re: the software - I know you have backups of the installation files, and that's fine, but you might want to save yourself the trouble and get the latest installation files for many of those (like AVG and Firefox). Otherwise you'll be doing a lot of upgrading, which in the case of older versions of Firefox means repeated whole installations.

Re: Outlook Express - really? That's going to be a pain. I hate how that program stores data. It's in a bunch of oddly-named files, usually in two or more folders. At least if you upgraded to Outlook, you could store everything in one PST. Then you could switch to something better like Thunderbird and import all the data (I don't think Thunderbird imports directly from Outlook Express, but I could be wrong).