Lumens are a measure of brightness. LCD Projectors will generally be priced according to brightness. Basically you should figure out what your biggest venue will be and then buy accordingly. In my case, I run little multimedia shows at events where screens are usually no larger than 15', and there are typically < 250 people. For the most part I've found my 2000 lumens projector to be more than adequate.
As far as portability goes, LCDs are by design. The old school 3 head projectors were heavy as hell. LCDs are now pretty light (10lbs).