I'm a big fan of the Dell Inspiron 17. In fact, that's that I'm currently using to type this response. I've had my Dell for almost 2 years and (knock on wood) haven't had a single issue with this machine and I bought it refurbished.
The one you've found on Costco will certainly do all the tasks your Aunt will be doing. Because of the screen size the battery that comes with the laptop will last 3.5 to 4 hours max. However, you mentioned this is more like a desktop replacement, so battery life may not bee an issue. If your Aunt plans on doing any traveling, I recommend getting the 12 cell battery, which I've gotten up to 9 hours running minimal tasks.
As far as price goes I think you're getting a good deal. I just spent a few moments on the Dell Outlet website trying to beat your price, but couldn't. And most on Dell's website come with Windows 8.
Bottom line, I think your Aunt would be happy with that laptop.