Liquitex comes in an "ink" version too. I'm pretty new to this, but I thought they were easier to work with than tube paint (the consistency seems to be pretty much what tutorials recommend you dilute regular paint to get). I haven't tried the other brands, but in general I'd suggest getting the liquid version of whatever brand you choose.
As for getting the eyes right the only tricks I know are the most basic ones: sketch with watercolor pen, paint both step by step at the same time (instead of finishing one and then trying to copy it), turn the head upside down now and then to get a fresh view... Those are the things I try to think of at least, but it's harder than it looks!