Hi,
It depends what you mean by 'edit' really.If you want to make changes to categories, stores, coupons then you do it from the back end of WordPress+Clipper.If you need to customize the way the theme works or the design then you will need css+php skills at least.
Is it set up for vendors to add coupons randomly?
Could you explain more please?
How can I set it up where I am the only person who can add coupons?
You can disable registration and you will be the only person who can add coupons.
Can I make this a "local only" site?
You can add coupons from your area only if that is what you mean.
The disadvantage will be that you will not have the graphic files (psd) of the theme so in case you want to make changes to the design you will need to recreate it in a graphic application (like PS).The psd files are structured and organized so it is easier to start from that point.