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...
