Are you sure you want a projector? Something like the Pico Projector might be what you're looking for. It's not too expensive, and it will definitely show. I personally don't use it or have seen it but after reading some reviews it seems the quality isn't that great and the screen may be too small.
However, if you can deal with those cons the projector is tiny and may possibly be the perfect fit for you. It takes in composite video (through an included mini-RCA adapter), which will be perfect for the Wii.
Personally, I would rather buy a small monitor with composite inputs for around the same price but if convenience is a factor, i guess I would give this a try.
EDIT: And be sure to read the reviews!