There's no easy way to answer how many sts you should cast on because that would depend on the yarn you choose and the needle you use. Check out The Knit Stitch
. It has a wonderful and very simple poncho pattern. You could that as a template for your own design. As far as cable patterns, have you checked out About's knit stitch library
? The 6/6, 6/8, and 8/10 are nice and simple.
That double cable you found looks good! I'll keep my eye open for any more...