For this price, I'm afraid all laptops you'll find will be quite large and heavy, which begs a question: how portable will it be? If you plan on travelling with it or having it installed in the car, I would look for something smaller, but it'll cost more of course.
Also, you may want to research the availability of navigation software for PC before you take the plunge. Most of the navigation apps have been designed for Windows Mobile/Pocket PC/Symbian, as these are the systems usually used on these small devices.
If it was me, I'd get a dedicated navigation system for travelling and then a basic desktop for home web surfing. You should be able to get the two for $750.