As I said earlier ... SOFTWARE FIRST .. HARDWARE SECOND
If you feel that you really do need to have a laptop connected to your systems then be sure you know which software you are using and then get all the hardware that works with it.
Case in point. I played with my Magellan 315 handheld GPS witha data cable (you can actually make your own). The troubleshooting tips in Capn Voyager showed that it was communicating with the laptop properly but Capn Voyager did not recognize it.
The solution. Buy Capn Voyager or whatever. Then determine which GPS units it works with and what Operating Systems, ports, etc... are needed. Then buy a laptop that supports this and a GPS that does as well.
Be warned that used laptops are used because somebody thinks they are no longer worth using. Refurbished laptops are refurbished because somebody thinks it was not working properly. That should tell you something.
Personally I can''t see playing with a laptop while sailing other than for interest and email. Mine was used primarily to watch movies when at dock with the family in the rain.
(and computer vendor)