Well, we're working on equality of Android and iOS users

meaning that we plan to release the iPhone version as well. However, the official release date for iPhone app has not been announced yet. You may want to subscribe to our
new versions notification letter to get to know at once when it's released.
As for the requirements, we are currently working on a system requirements calculator that is to be published on our site. With its help, you will be able to calculate approximate processor, HDD space, network bandwidth, graphics card requirements, and get our recommendations.
Until then, we're going to have to do this the good ol' way
What specs will the cameras have? What video format do you plan to use?
MJPEG has better quality, lowest CPU load, but can take much of space (but, if fps is not really high, HDD space taken will also be quite acceptable).
H264 or MPEG-4 offer compression that may help save space but give too much of a load on PC with Xeoma, and therefore increase the requirements for stable work. Your choice here will determine the required specs. As for the card, well, it must work with standard drivers for the chosen OS.