You can get an N800 on eBay for < $150. But unless you like tinkering to make it do what you want, I suspect this isn't for you. If Streets & Trips is Windows software, it won't run on a Nokia Internet Tablet, although it has other mapping software, e.g, Maemo-mapper.

I wouldn't be surprised if you'd be happiest with a cheap GPS unit, augmented by a Palm or home use of your computer.

These Tablets are rugged, small, flexible, and pretty much all software for them is free.

Look here for a general idea:

http://en.wikipedia.org/wiki/Nokia_N800

http://en.wikipedia.org/wiki/Nokia_N810


Good luck with your business!