tried various things..
Is this possible?
what files need to be updated?
Creating page templates in general is covered here:
and if you scroll down a bit from there, you'll find:
Individual Pages can be set to use a specific custom Page Template (a PHP template file, e.g. snarfer.php) you create within your Theme (see Creating your own Page Templates below on how to create a custom template). This new Page Template will then override the default page.php Page Template included with your Theme. See What Template is used to Display a Particular Page?, below, to find out exactly which Template will be used. But read what follows first, so you understand the answer :)
WordPress can be configured to use different Page Templates for different Pages ....
As long as you follow those instructions, anything is possible.
If anyone is looking for some extra income I'd entertain an offer.
Take a look at the rules regarding offers for paid work: