The main reason to rank for an informational keyword is to help drive traffic in order to rank for keywords that do sell. Although some people do convert from an informational keyword through rating and review sites but I do not have a percentage for you. I guess its also useful for making Adsense revenue...

There are only 3 reasons people search, for information, for services and I can never remember the third one, for products? Not really sure. So if you can be an authority for each type of search its kinda like branding.

Of course you can, but your conversion rate will obviously be lower. How much lower depends on how good your ads and/or marketing copy are, how relevant the product is, and the demographic you're targeting.

I'd expect at least a .33% conversion rate with informational keywords, but I've also had it over 1%.