1. Yes they will (unless there is a custom cooler installed, not usually the case)
2. Generally yes they will have a shorter life, however, this is negated by the fact that most GPU manufacturers warrant there cards for 3 years and some warranty for life of product. So even if there is an issue with failure it can be repaired under warranty.
If you dont have OCing experience and dont intend to give it a try (with obvious risks) then a factory OC'ed card will be the best option especially if stock cards are similarily priced.
As you suggested a good air flow case will partially negate the potential heat increase of the OC'ed cards, of which the Antec 900 is a good choice.