Try and get the best true resolution you can afford. Most of the cheap projectors will have a resolution of around 640x480 or 800x600 even though they will have "HD" or "1080p" in big letters which means the accept the hd signal and down convert it to the low resolution of the lcd panel, so for watching football or golf you will not really get good detail on the players or ball.
With the lcd panels inside them they usually have a poor refresh rate so you will see motion blur when a ball is kicked for example.
Saying all that, if you can get one for £20 and are not happy, if you have kids they will love watching cartoons on a big screen at the lower resolution.