I have a site that uses an external mail exchange service (rackspace), is it possible to set this up on the Managed WP Hosting?
Also, are there any issues with using plugins such as WP Rocket (cache plugin) or the iThemes Security?
Sure you can use remote email, and neither of those plugins would cause issues. wp rocket would conflict with LSCache which is superior. I'd recommend you not use wprocket.
On a VPS or Dedicated server you just select remote email server when setting up an account. For WP hosting you may need to contact support, but it all depends on where you set your nameservers. You could manage these settings from your domain registrar, but if you'd prefer you can just set your nameservers to point to your KH server and KH support can easily handle that request.
Yes, WP updates can easily be disabled in WP Admin by adding a disable updates plugin or add a line of code to wp-config.
http://www.wpbeginner.com/wp-tutorials/how-to-disable-automatic-updates-in-wordpress/
Is that something support would have to handle or are those settings available to the user?