I'm looking at migrating my site onto Wordpress
Migrate from what, html (I do not know what type your site is)?
Free Plug-ins.
Google XML Sitemaps (automatically builds then rebuilds your sitemap, everytime you add a post or page).
W3 Total Cache (Vital. It cuts the page load down to half. I got a site from 11 seconds down to 4 using this).
All in One SEO Pack or Yoast (but don't go too mad on the SEO).
Akismet (anti-spam).
Redirection. Does your 301s, 404s etc.