The Atari Portfolio was one of the first, if not THE first, pocket-sized IBM-compatible computers ever to reach the market. It either predates or is contemporaneous with the earliest IBM-compatible palmtop offerings from Hewlett-Packard.
The two main limitations you'll find with the Portfolio are that the screen size is non-standard (though it can be configured to emulate an 80x25 display), and that it has a very limited RAM size. It is possible to upgrade the unit to 512K of RAM internally (third-party companies were doing this at one stage); once I get a moment to investigate my Portfolio's mainboard, I will determine what parts are required for this. The removable memory cards are proprietary and no longer in production. Apart from these limitations, the Portfolio can run a good deal of commercial MS-DOS software. It comes with a word processor, a scheduler, a contact manager and a Lotus 1-2-3-compatible spreadsheet built-in to ROM.
My association with the Portfolio only goes back to 1994, when I worked part-time for three months at a small, disreputable computer dealer in Melbourne, Australia. For some reason, we had two Portfolio units lying around the store (it was already a has-been machine at the time), and I carried one in my jacket pocket for a couple of months - I fell in love with it! Unfortunately, the store owner wanted more than I was willing to pay for the machine, so I left them both behind when I moved jobs.
Secondhand Portfolio computers and accessories seem to be quite plentiful on ebay, and the only problem you are likely to have is a cracked LCD. Because the Portfolio has no backlight, few moving parts (apart from the keyboard), and its electronics run very cool, the units that are currently functional are likely to remain that way for quite a long time.
When I received my Portfolio, a couple of lines on the LCD were not working. This is one of the most annoying problems to fix! In order to get in and reseat the bad connection (which is the cause of this problem), you need to prise the faceplate off the LCD, revealing two screws, remove those screws, carefully pry open the casing (it's held together with plastic clips), unscrew the LCD controller, bend open the retaining tabs that hold the LCD onto the driver board, and wiggle it all apart. This process can only be carried out a couple of times before the retaining tabs work-harden and fall off.
The Portfolio seems to have been best supported in the days of proprietary online services like CompuServe, and not all of this information has been brought over to the Internet. The archive below contains all the reference information I have found on the Portfolio thus far, in one tasty ZIP file. It includes some software, a FAQ, some electronic newsletters and their accompanying software, and a couple of other handy bits of information.
Because the Portfolio is basically an old PC/XT (albeit stripped-down), the idea of "emulating" it doesn't make a lot of sense, and I am not aware of a specific emulator.