You go with Lenovo ThinkPad and you won't be disappointed. They cost more but are the primo brand for laptops. I spend the extra on it since I interact with my laptop every day.
You can also buy a business class Dell which has improved greatly over the last couple years. They are almost as expensive as ThinkPads now.
Acer makes good "bang for your buck" laptops. Hit or miss on how long they last, just depends how you treat it. But you can get a working one for under $500.
Stay away from Sony, HP/Compaq, or any name you haven't heard of. They offer all kinds of bells and whistles but the inner workings are junk.
Or you can get a Mac, but I know nothing of those. I know no one who is disappointed though and many who refuse to use anything else.
As for air card for connecting to a cellular network, one of the big carriers is offering a plan now where they bundle mobile phone service and air card service into the same package. This is a big step since it usually costs $40-$60 more per month to have an air card. You'll have to shop that one, I don't know who is doing it. It's rare that you need an air card these days with WiFi in every public area. Unless you are trading on the beach :)
The model of any laptop you get can simply be based on the features you want. It's more important to pay attention to the manufacturer.
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