https://bitcoinwisdom.com/bitcoin/calculator is a good calculator. A good choice is to set the difficulty increase to around 20%, however, it cannot always increase 20% therefore, it should only be used as a rough estimate.
Renting out mining hardware profits provided that people want to rent them either to test their pool or mine for fun. Wise people wouldn't buy them with the intention of profiting.
20% used to be the minimum increase in difficulty per 12 days, man. Those were crazy times! If ASICMiner really sets up new hashing data centers of 60 PH/s, the difficulty will skyrocket, man! All the other miners will have to shut off their gear, man!
The 20% was when ASICs were shipping at a super fast rate. I'm seeing more people receiving their miners so you can expect a 10%-15% increase or so. With Bitcoin gaining recognition, more people would start to mine, pushing the difficulty up.