The car and portable units are different, but the biggest difference that you will notice is the screen size. The car models are much larger and far easier to read.
If by "in the box" you mean inside a metal container, it will probably lose the signals. When you stop, it will have to reacquire the satellites, which may happen quickly, or may take two or three minutes. It's hard to say exactly.
If the box isn't metal, the unit will probably be able to keep tracking the signals while it is in there. Just be aware that GPS signals are very weak and any kind of material between the unit and the open sky will attenuate the signal to some degree. Some types of material are worse than others. Wet anything is usually bad. Cheaper units are much more prone to losing the signals than the more expensive models. The attitude of the unit makes a difference, too. A unit laying on its back normally will not do as well as one held vertical in your hand, for example.
I suggest buying a quality portable unit designed for outdoors. If it doesn't provides acceptable performance in the car, then buy a car unit, too.