Most projectors use liquid crystal display technology. Yo* need to be concerned about resolution, the amount of detail that will be revealed and it has to match the computer that it is working from. VGA (video graphics adaptor) requires 640x480 resolution, S-VGA 800x600, XGA 1024x768.
Brightness is measured in lumens: 600-800lm work well with a 100in-150in (250cm-400cm) diagonal screen in a dim light, 1,000lm are needed on larger screens, 1,500lm or more are needed in bright conditions.
The weight of the projector will be important if you are going to be moving it around.
Yo* must also consider whether you will use only PAL television signals (as used in most of Europe).
Check the video resolution. The higher it is, the sharper will be the image.