How to calculate the the amount of all Peercoins(PPC) in circulation? According to Wikipedia:

http://en.wikipedia.org/wiki/Peercoin

the reward is halved every time the mining power increases by x16. So does that mean that you need historic data on the mining power in order to calculate how many coins there are?

Also, does anyone know what's the initial reward and difficulty? I can't seem to find those either
According to Wikipedia:
  
  http://en.wikipedia.org/wiki/Peercoin
  
  the reward is halved every time the mining power increases by x16. So
  does that mean that you need historic data on the mining power in
  order to calculate how many coins there are?


No you do not need historical data on mining power, but you can calculate out the historical data with a simple calculation used by the entire ppc chain to get an idea of how much hashing power there has been historically. (but that's not the question you are asking).


  Also, does anyone know what's the initial reward and difficulty? I
  can't seem to find those either


Block explorers are your best bet. ABE based explorers like cryptocoinexplorer have some nice API functions. using http://ppc.cryptocoinexplorer.com/chain/PPCoin/q/totalbc I am told there have been  21019084.894021 PPC mined currently

the first block with a reward (block 1) hash is 00000000000be4e024af5071ba515c7510767f42ec9e40c5fba56775ff296658

http://ppc.cryptocoinexplorer.com/block/00000000000be4e024af5071ba515c7510767f42ec9e40c5fba56775ff296658
"
2499.75 Coins generated
Hash: 00000000000be4e024af5071ba515c7510767f42ec9e40c5fba56775ff296658
Previous Block: 0000000032fe677166d54963b62a4677d8957e87c508eaa4fd7eb1c880cd27e3
Next Block: 0000000000fdfc564b25df5858ad9f5e3378d9102a22d5c785f9ab92ec3a304a
Transaction Merkle Root: 1bdae84eff34e15399335fc2a48c70fa6b0b9caf972e38b0e3bda106d223f668
Time: 1345400356 (2012-08-19 18:19:16)
Difficulty: 256.003 (Bits: 1c00ffff)
Nonce: 1915373966
Transactions: 1
Value out: 2499.75
"

So there were 2499.75 coins on the first block.
The first mined block had a difficulty of 256.003 

as always, google is your friend. Check it out