An 800x600dpi resolution is usually sufficient for a good presentation. If you're going to be carrying your projector through airports or on trains, look for the lightest possible solution. A good quality carry case is essential, as portable projectors are likely to spend at least part of their lives in transit. Don't be distracted by motorised zoom and focus controls. They are just gimmicks that add to the weight.
Lenses are often irreplaceable.
Make sure any projector you buy comes with a lens cover, and use it whenever the projector is not in use. Projectors often have built-in speakers, but these are usually next to useless. Budget for external speakers if you need audio in your presentations. Spend some time configuring your new projector with your notebook before you go out on the road. It will be time well spent, as the projected image will almost certainly look better with a bit of tweaking.