Greetings also from New Zealand :) In answer to your questions:
1. If you're specifically wanting to create subdomains for various regions similar to what you see on
this site from our
customer showcase, then you may like to look into a
WordPress multisite setup. The theme itself can manage various locations within the same country on the one installation, but you'd need to use the search specifically to find those listings.
2. Clients can edit their own listings, so if you wish to allow them the ability to edit the listing description or create a field specifically for offers then they can. The theme itself doesn't have a coupons/offers feature for businesses as such though.
3. Vantage does not offer this functionality by default, so it would require customization on your part if you were looking to add this to the theme. AppThemes does not offer customization services, however we do have a
certified partners page where you can find developers for hire who are experienced at working with our themes.
4. Yes, you can create paid of free listing plans whenever you like.
5. The theme uses the native WordPress categories functionality, so you can add/remove/edit categories and subcategories as you wish. Thanks.