The first question you have to answer is how much will the rig cost, how much will it cost to mine, and how much will the rig mine? Spending money without answering those questions is foolish.
Second, leasing a rig doesn't make sense. Basically, you don't want to lease it for less than it mines (because you could make more money mining), and nobody with a brain will lease the rig for more than it will mine.
Generally agree with your points, but I would like add a little bit info on your very last point.
In fact, I do know a few persons renting scrypt hashrate at premium to mine some new-born coins and hoping it will hit an exchange (and the price will go up significantly).