Serial Ports are able to speak up to baud rates of 115200 standard!

Does not matter which laptop you get as you'll be restricted to this baud rate anyways.

Better option is to select your ecu (either based on either your own research or your tuner of choice) then get a laptop.

Most ECU programs to date would do okay on a pentium iii 500 (dont hold me to that) after all its just displaying data as quick as the serial port is able to send/recieve.