Using a 301 redirect will help you retain the SERPs for the pages you already have and be a good bridge from your old to your new content.
Don't be overly afraid of CMS's though as many now have excellent
SEO integration, such as Wordpress and Joomla, though with some you have to do extra bits of work here and there - but it's certainly not a no-go.
For Andy
SEO, i guess it would depend partly on the build nature of your redesigned site as to how and if it would affect your rankings. Certainly if you had a multi-level tables based layout this would seriously drop your ranking compared to a clean CSS based one.