I make extensive use of projectors and laptops, but I found a GREAT tool that makes my life super-easy - it's called an AveryEpack - it's a little box that holds Powerpoint presentations and other images. It has two outputs - one RGB port that connects to any monitor or projector, and a TV port that can output to any TV with a standard video port. 

In summary, that one little box creates the image and can shoot it to a display device. If your site(s) has a large enough TV, you don't even need a projector. It's really awesome... I've used it a couple of times with great success.

Here a link where you can check it out - the whole thing is a big $200!

If you're really set on a laptop and projector, you can grab a cheap laptop on EBAY for roughly $400. As far as projectors, you'll want something with 800-1000 lumens of brightness at least. Don't get something too old or too dim as it might be too dim to use in really bright rooms.

Editors Note:
The price I found online at CDW - 

AVerMedia AVerEPack EPACKDS0 AVerEPack $199.99
Standalone presentation device with up to 640 x 480 resolution

AVerMedia AVerEPack300 Digital Slide Presenter $249.99
EPACK300 AVerEPack300 1024 x 768