You are more or less out of luck on the decoder chip, methinks. They are easily available, more or less; but aren't designed to interface with computers. They are OEM chips designed for inclusion in manufacturers designs. There was a period, back when 400mhz was a pretty extreme CPU speed, when DVD decoder PCI cards were available, something like H.264 coprocessors in certain modern graphics cards.
There is one PCMCIA version available
http://www.baber.com/laptops/dvdtogo.htm but it seems to be the last of a dying breed. You would probably need one of those, along with a DVD drive. This could either be added as a second PCMCIA peripheral, or you could open the laptop up and have a go at replacing the internal optical drive(if any). If there is no internal optical drive, or if it is pre IDE or otherwise not replaceable, you'll have to go with PCMCIA. PCMCIA disk peripherals are also becoming somewhat rare, and may be difficult to aquire.
Battery life may also be a concern. If your idea of "portable" is "easy to carry from place to place" you'll be all set; but I'd be terribly surprised if the battery on a box of that vintage is up to the task of playing an entire DVD in one go, much less with attached peripherals. You'd have to re-cell it, at least.
Once that is done, you'll need to attend to the software. I would ordinarily advocate Linux; but if we need all sorts of oddball outboard hardware on an older laptop, that is probably asking for trouble. So you'd probably end up using an older version of Windows.
The punchline seems to be, unless you can find considerably lower prices for the necessary parts than I could, that it would be possible to do the retrofit, certain conditions assumed; but quite uneconomic. Your choice either way.
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