You have mentioned some good tips about adding to the values of websites, mainly through content writing, which I also believe has become more significant. Thousands of webmasters are today focusing on building more links for their websites but they have forgotten one essential fact and that is, at the end of the day, human visitors will be visiting their sites and they would like to find some quality stuff to review and study.
About the number of words inside each page or article, that actually depends on the subject you are talking about. Maybe even 250 words is enough if you manage to write pretty well and pick the accurate grammatical and structural units. Using a few directory links might also be nice, especially the ones that come from authority directories, high PR portals that thoroughly cover the niche in which your website is active.