Use a laptop with the fastest processor that you can afford. If you are using it only for panel configuration purposes then the HDD size is not so critical but i would suggest at least 40gb.
Many existing upload leads are for use with serial ports. however this will change with new software versions using the more popular USB ports. If you can get a laptop with both serial and USB ports then so much the better.
Even though it is becoming less popular, I still feel it is worth having a floppy disk drive. A CD drive is a necessity.
The operating software loaded onto the laptop is critical. Many existing configuration programmes cannot handle Windows Vista software. Try to get Windows 2000 or Windows XP in any of variants. Software configuration writers will eventually 'debug' their programmes to work with Vista, but it may take some time.
The most important thing is you will be transporting the laptop between sites and using it on sites. It will have a hard life. Dust, vibration and knocks must all be taken into account. maybe a cheap refurbished second-hand model could be a good idea.
Finally, you will often be standing in a front of a panel balancing the laptop on your arms - size and weight can also be a critical factor!
