Welcome to the Bunnings Workshop community @NicolaMcMahon. It's wonderful to have you join us, and many thanks for the question about restoring your door.
Varnishing or painting would both be viable options to restore the look of your door, and likely the decision of which to go with will be down to personal choice. Varnishing retains the natural beauty of the timber, whilst painting allows you to add a dash of colour to your home.
If there is discolouration to the varnish or the timber, the re-varnishing process will require you to sand out the marking from the surface before applying your new coat. The painting would be easier as you'll just need to thoroughly clean the surface with Sugar Soap and flush it off with clean water, waiting for it to dry before proceeding. Apply an undercoat of Dulux 1L PRECISION Maximum Strength Adhesion Primer White, and then apply your choice of topcoat colour. I'd suggest Dulux Aquanamel for the finished topcoat.
You should find these step-by-step guides helpful: How to paint your front door and How to restore wooden furniture. We don't have a How to varnish a door guide, but I've checked the steps in restoring wooden furniture, and they are much the same. You don't need to sand right back to bare timber as in the guide, just enough to remove any discolouration.
Changing the door lock style might require you to alter the cutout for it in the door. However, that would open up the option of choosing something from our range with updated styling. If you want to swap for a similar style deadlock and handle, you'll find these guides useful: How to install a door handle and How to install a deadlock.