You are correct to say that querying more than one source of content for website search purposes with Lucene or Solr as used by Sitecore is very hard, or not possible.
You are also correct to say that a unified index is the solution to this problem. There are multiple unified index products that could solve this problem:
- Elastic Search. No integration in Sitecore. Runs on your premises, you manage the servers and services. You have to code everything to index and query Sitecore items and third-party data.
- Coveo for Sitecore. Commercial product fully integrated in Sitecore. Hosted in the cloud and offered as a service (SaaS). No code required to integrate in Sitecore or crawl third-party data as it has dozens of connectors for enterprise grade third-party systems.
In both cases, you probably do not want to switch from Lucene/Solr to the chosen unified index. Lucene/Solr is used extensively by many Sitecore administrative tools and manages many indices. Switching all of them would be a tedious task.
I recommend you index only the required Sitecore items and third-party data in the unified index and use it for your website search needs. Leaving Lucene/Solr in place for out of the box Sitecore functionality.
As for the Elastic Search installation guide validity for Sitecore 9, I do not know. You are better to leave a comment to the author.
Disclaimer: I work for Coveo and I am not against other search solutions. Each solution fits different needs.