I’ve never needed to use a Page Builder, so I can’t offer any advice or expertise on this subject. The Page Builder that you refer to in 2.3.1 is currently an exclusive feature of the Magento Commerce Edition. This is the “Enterprise” version of Magento Open Source (which I believe we’re all using here). That’s probably why you can’t find any additional information about it, because the people purchasing the Commerce Edition license aren’t the kind of people who do YouTube videos on it.
I own both Ultimo and Porto. I reckon they’re completely on-par with each other. The only real difference to pick one over the other is how it looks. I personally prefer the look of Porto.
Just be a little wary with outsourcing work, especially with more complicated projects. I had a small rant about that in this post (Where to find a Magento 2 developer?) 