“Is it as simple as me jus

“Is it as simple as me just setting up the template in HD, then choosing to render as either HD for BluRay or SD for DVD?”

Yep, it is that simple.

“Setting up the template in HD for editing but then rendering to SD will not cause issues will it?!”

the only issue you might run into is letterboxing because of different aspect ratios, but you can fix that by altering the SD frame size while rendering.