I would like to build a very niche expert platform for the Spain & Latin market and am trying to see what theme would allow for voice but also text questions to be asked, all pre-paid. Many thanks in advance.
Well first, it's important to point out that this is not a Wordpress site. It's a web application or SaaS. Perhaps it was built with Ruby, Python, or it could be using other JS frameworks/languages. There are many reason why you would not use WP for a site like this, one of those reasons is: Scale. WP sites do not scale well. I would start there. I would research building a web app vs. Wordpress. You will find much insight in regards to scale. This is a great article to introduce you to WP scale and what issues you may run into: https://tommcfarlin.com/scaling-wordpress/
You are on the right track. It's not what theme, what plugin. That is the key.
You can use a free plugin like BuddyPress to give WordPress social media/profile-esque functionality. I would look into getting that plugin modified to do what you want. You may be able to get it close to what you want with enough tinkering without having to dish out some money for a developer.
You could most likely integrate a payment gateway with BuddyPress to achieve something functional.
If you would like to build a niche expert platform you have to know the tool on which you can create a great niche platform please look for the plugin called cart flow and i hope you will easily create a niche on this plugin.