I’m not sure what the topic you linked to has to do with what you’re trying to do. The standard install instructions are here: https://github.com/discourse/discourse/blob/master/docs/INSTALL-cloud.md.
I recommend against trying to use Discourse with Plesk, though here are some instructions: Installing Discourse With Plesk Onyx (Ubuntu 14.04). For $99, I’ll install Discourse on its own Digital Ocean droplet. (+ $50 to turn on https, if you want me to do it for you). I’d not install it on Plesk for less than $500.
Migrating your data to Discourse is a whole other kettle of fish, but it’s well documented: Importing from phpBB3. For an estimate one would need to know how many users and posts your forum has. I typically charge $500-1000 for an import, depending on size and how much help you want handling the transition.