What is the purpose of PoS minting difficulty? How is it calculated? How does it factor into the minting block selection?
The PoS difficulty relates to the target per coin day. Just like the PoW difficulty relates to the PoW target.
To find a block you need:
hashProofOfStake < CoinDayWeight * TargetPerCoinDay
here is a wiki how difficulty is calculated. Keep in mind that peercoin is a fork of bitcoin: https://en.bitcoin.it/wiki/Difficulty
The key difference between mining and minting is using a nonce and a timestamp. Thats it. A nonce is a random number you can generate millions per second while a timestamp just once a second.