I actually think a laptop would make a perfect server with light traffic. 1) It doesn't take up much space, 2) It doesn't produce much heat or consume much electricity, and 3) it will not be very noisy (the only P100 Portege is the Portege 620CDT which doesn't even have a fan).
If you are worried about heat, then prop it up in the back and keep it in an area where it will get plenty of fresh air (i.e. don't sandwich in a bookshelf). Otherwise, don't expect much performance wise, but hey, it's a small basic server right?
As far as a nic card, steer clear of Cardbus nics, as they won't work with that older laptop. I would research the nic you are interested to make sure that linux has support for it. Still have found drivers for my Proxim 7200 wireless nic.