If you're offroading, the handheld/auto units are about useless. Offroading with downloaded (free) photo maps from the USGS site and (free) software called USAPhotoMaps is great. I popularized that setup here and more and more of us are getting it.
I use a Panasonic Toughbook CF-27 laptop and it's adequate for that. You really don't need the best laptop for GPS mapping, but you do want enough hard drive space to store the map data -- which can get large. I have several gigs of photo map data for all the sites I offroad, as well as both Garmin and DeLorme mapping software.
I use a dedicated GPS module also from a police car for my setup, but I used to use a Garmin eMap. There are USB GPS modules available for about $100 or so that work fine with all the above software. Make sure your laptop has both USB and regular serial ports and you'll have the most flexibility.
If you can get a GPS that can use an external antenna, that's your best bet. I've used GPS's on the dashboard, in the window and it's never optimum. You'll eventually hit a situation where you lose enough sat locks that you can't get position fixes. Get your antenna outside if your need for navigation demands full-time performance.
If you want to see more about the USAPhotoMaps software, then search USAPhotoMaps on this board, and go to http://jdmcox.com
to download a free copy to play with.
USAPhotoMaps is almost useless for street navigation. It does not support turn-by-turn routing as the commercial street navigation packages do. You can buy used, older revision copies of navigation software at ebay or half.com for like $10. It doesn't have to be a "major investment".