What do I need to know about mining difficulty before starting a bitcoin mining startup?
If you want to extrapolate what the difficulty will be in the future, here is a chart you can look at:

http://bitcoindifficulty.com/

The big thing about difficulty is that it's hard to predict. In general, it's almost always increasing due to better hardware, but it's hard to predict how much it'll increase because bitcoin hardware increases with innovation, something that's intrinsically hard to predict.

Generally, there's a lot of guesswork involved and forecasting to do as a businessman.