GPS guidance for spraying is going to require a completely different program from what most of us use here. You will probably have to search for that software on your own since it's so specialized.
As for web browsing, chances are that while you're out in the field wifi will not be avaliable so you will have to have an EVDO card or the like from one of the cell phone carriers ($50 a month at a minimum).
Due to the nature of this hobby, you will need to do your own research on product and interfacing components.
There is a ton of information to be had on this forum and many of us have taken it upon outselves to try and make this information easier to find and to understand. I'd suggest utilizing that work to make this whole process easier on yourself. You can start with the FAQ link in the nav bar at the top of the forum as well as the FAQ Emporium sub-forum that we have here.
I would also recomend checking the store here at
http://store.mp3car.com. There are quite a few systems designed in there that allow you to just pick memory and hard drive size which makes it much easier on you. Screens are avaliable there as well.
Any specific questions should be directed to the forums, all we ask is that you do a bit of preliminary research and to help yourself as much as possible. In the long run, you'll see that there is a great benefit to really learning about this stuff if you're going to actually be using the PC on a regular basis.