Buy a cheap camera and invest in a pair of side lights.

If you want to experiment, get hold of a large wire framed lamp shade and strip the covering off it, or get one from a craft shop without any covering - place the camera on the top pointing down through the opening, mark off the picture area at the bottom. Place the lamshade on the document and fire away. For large documents, move the lampshade along, taking a series of slightly overlapping photos and then stitch them together in Microsoft's ICE.

You wouldn't really benefit from an expensive camera, as long as it will focus down to the distance you require, you wouldn't benefit from a resolution greater than 8 megapixels and the file sizes will be enourmous for a large plan. Self-timer or remote shutter release for rock steady shots would be a bonus. See what they've got in Cash Converters or eBay.

Flash would be no good, the light would bounce straight back up through the lens, you'd need to fit some kind of side lights, angled down at about 45degs so that the light is reflected off the document and away from the lens. At least two, for even lighting.

Just a thought. It might work :)

I'm wondering if you'd get better results using a film camera and scanning the reulting slides or negatives.