Yeah, I think IBM builds the most solid laptops. Dell might present a better value, but I've read a few stories of poor build quality--still, fora few hundred $$$s in savings, you could probably deal with a few squeaks. Just make sure you get Win2K bundled (read one of the many open threads on Dells in the Hot Deals forum for details), as WinME seems to be the culprit for myriad problems. And if you go Dell, opt for the 3-yr warranty. For spreadsheets, you probably won't need more than a P3-600, which comes with SpeedStep for power saving, and draws less power than the its SpeedStep brethren. Go for an active-matrix (TFT) screen. Get as little RAM included as possible, then buy more cheaply online (at Crucial.com)--128MB total minimum, the more the better (and the less you'll access the HD, good for extending battery life).
First, go to a local store with your dad to see what screen size/pointing device he's comfortable with, then start price shopping once you've got a basic idea of what you want. I myself consider the IBM and Sony laptops the most desirable, but I don't have great experience with either, and uness you buy online, you'll probably incur a hefty penalty with state tax. That's why Dell (the Inspiron 4000) may end up being your best bet.
And read the reviews on CNet, ZDNet, and PCWorld for a general idea of laptop faults/quirks/desirables.