As I'm sure you know - the usb / serial port is normally only used for moving the mouse around / clicking for the next slide. If I was them, I wouldn't bother with this feature - buy a separate PC remote if they must. I would expect the speaker to bring their own if they needed it.
If you're sending the picture down the USB cable then you are going to get a poor resolution and poor refresh rate. Thats what HDMI / VGA / DVI is for. Make sure you include a VGA port - all laptops can output on VGA (maybe with an adapter from DVI) but only newer laptops can output in HDMI.
Also get one which has a composite phono / RCA input - so they can watch DVDs etc if they want (although you would also need a separate set of speakers).
Most presentations are still written on PCs in a 4:3 aspect ratio. Home cinema projectors are normally wide screen. If you can find a projector that is 4:3 it would be better, but they are hard to come by at the bottom of the market. 1024x768 should be the minimum resolution - but the bigger the better. Most of the projectors in that price range wont make this.
For an indoor presentation - you would want a minimum of 1000 lux output - but the more the better, especially if its a bright room. If there is sunlight coming into the room, you'd be better with 3000 lux - but I suspect that would blow the budget. You will never be able to project onto direct sunlight. Those Pico projectors are 8 to 25 lux so not suitable.
Ignore 3D - everyone watching would need expensive goggles - just not worth it.
HTH
RR