The power (lumens) you get should be proportional to the size screen/room you need to project in. I don't know the direct corespondance though. Buy a projector which can fill the biggest screen you think you'll need in the next few years.

Also, will you be traveling, taking your projector to demos at other offices? There are many small, lightweight projectors which are good for small offices. They're worth their (light) weight in gold when you can bring it in your carry on in the plain.

I have never had a problem with my inFocus projector, an LP-180 from five years ago. Tiny thing which my staff members bring all over the country. Saved $thousands in rental projectors at conferences.