I would look at hiring someone who is knowledgeable with the composite. We have seen many poorly installed composite decks that really look bad. In addition if there is a warranty problem then using a recommended contractor will help.

Yes, yes, yes, buy and pay for the material yourself. If you ever have a problem with your deck the first thing the manufacturer wants is a copy of your sales recept. Try chasing down a contractor in 5 years and getting a copy of that. Also it keeps you from liens if for some reason he does not pay his bill to the lumber company. As far as where to look, if they are listed on the web site they are suppose to meet certain guidelines but you don't have to use just those people. The biggest thing I tell people to do is go look at what they have done. Talk to there customers, ask about problems, time to finish the project, and please make sure you see more than one customer, everyone has a relative they have done work for.