Updating the core from an earlier version to latest version.

Hi there,
I need to update a website from drupal core 6.13 to the latest release 6.26, plus numerous modules and a theme requires an update.

Do I need to install each update progressively i.e. 6.14 > 6.15 > 6.16 etc or can I simply install the latest update over 6.13?

Similarly some modules have multiple update packages, should these be done progressively or jump to the latest update?

Before I do any work on the server I'll be creating a test environment on to my workstation. Will it be possible to create a test drupal environment from 6.13?

thanks.