There's no "best" answer here, but you can approach it a number of ways.
For trendy/future-facing, an isomorphic JS stack (Redux, React, etc.) could be a good move. The architecture is scalable and — if no one on your team hacks around the implementation details — it's easy to maintain (and onboard new team members).
You can also use WordPress as your CMS, but that can get weird when scaling, and isn't the most developer-friendly solution due to WP's wonky architecture. You can unfuck it somewhat using something like roots.io (Trellis/Bedrock), but at the end of the day it's still WP, and those quirks are part of the price you pay for the ease of starting up.
Another option is to use WP as a CMS, but read the content from the REST API and generate sites with other front-end tech. I recently built some tools to generate static sites (for deployment in Amazon's S3/CloudFront) that pull the WP data as JSON and then generate static pages with my preferred front-end tools, leaving the weird PHP templating out of it entirely.
To take a crack at a tl;dr: talk to your development team about how you want this to scale, and where you see it going in the future. If you don't have a development team that can make those calls, talk to a specialist to help lay out a roadmap.
If you need help with this, I'd be happy to take a look at what you're planning and offer some insight into how it could be built to scale and grow painlessly (or at least as painlessly as possible). Send me a message and we can talk details.
I would absolutely agree with Jason. There is no perfect answer. There are only options. And the best option is to use something that you or someone in your team already knows. If you know PHP. Why not use it to your advantage. If you know WP and are able to build it this way - good for you - just get started. Get users on the platform. Have people use it. And worry about the next step once there are users.
In the long run. I personally would build something on top of Node.JS with a front end framework of your choice (react.js, angular.js or ember.js are good options).
I hope this helps a little and keeps you motivated. In the beginning, it's all about actually bringing the idea to market. Actually launching and then see where you want to develop your own new solution.
And as Jason mentioned: even if you stick with WP as your MVP. Once you want to start developing it further, you can still use the WP REST API to read data from WP and start building a new interface around it.
Good luck and have fun on that journey!
In case you have chosen WordPress for MVP it’s better to go with existing solution for community, like BuddyPress. While it’s ok for MVP except issues as the userbase grows. At some point you will need to migrate data to some other platform and it might not be the easiest task.
As to what could be used for the tech stack - I would recommend to go with technology your developers are familiar with. Please note, it will be easier and less expensive to find developers familiar with php/mysql than, for example, JS stack. So keep that as a consideration.
There are few scripts/platforms for communities already available. But the quality of code/architecture differs.