You won't find many modern day laptops with a builtin serial port so you'll
need to go with USB or get a Cardbus/PCMCIA serial card.
As far as laptop brands go, I'd go with IBM Thinkpad. These are rugged, reliable units
and can be picked up second hand quite cheaply.
For some cheap new and second hand laptops go have a look at
http://graysonline.com.au