Hi Kish,
I looked on the Rohan site a couple of weeks ago, during their 20% off everything sale and couldn't see the poncho, has it reappeared?
As to the string you use, I'd make a decision when you get the pocho, you want something strong enough but as light as possible yet not so light/thin that it damages the poncho so when you see the poncho fixings you can decide. As to the length well, try it out ad see what works for you, for my poncho tarp I got ready made spare guys from a shop and they must be about 3.5-4m long all told incluing the ajustment to tension the tarp.