If you are going to project outdoors, then you need to ensure that your projector (beamer) is protected from the environment. However you also need to address a few other points before making your investment.
Whilst choosing a projector:
Will the projector be bright enough for the typical lighting for the time of day it will be used?
What will the image be projected onto? The projected image should be onto a light coloured surface; obviously white is best. A screen is not always used for outdoor projection.
Buy a lens to suit requirements.
Whilst Choosing the Outdoor Projector Enclosure:
What will the orientation of the projector be?
How will the equipment be mounted?
What are your cabling requirements? For example, cable entry, HDMI, fibre optic…