Something else to point out that I did not see mentioned above - for your first three wives, aim at any noble daughters you can either gain direct political advantage from (Uesugi, Yamana, Ouchi, etc), or any with a +Fertility bonus.
I want to point out that the AI will definitely choose wisely in these regards, too, so such juicy marriage targets get sucked up pretty quickly once they're available.
Come to think of it, the AI does a good job at a few things in Sengoku. Utilizing smart marriage arrangements, Plotting (when it's possible currently), reinforcing with fresh levies, etc. It does some things pretty well.