Best alternative to Google Custom Search? / for searching different installations

[Kass]

Hello all,

I use PW for my blog and a different installation on a subdirectory for e-commerce.

I want the search form on my PW blog to be able to include search results of the subdirectory shop, and the search form on my subdirectory site to be able to include search results of the blog. I am thinking about displaying search results under different tab headings so that customers can filter them - e.g. Shop; Blog Posts; Services.

Originally, I was going to implement Google Custom Search to do this, as it can search multiple websites and is quite common. But because it is ending its paid solution this year, I am looking for an alternative. Am I right in thinking that Elasticsearch would be the best solution? 

Can somebody advise on the difficulty of implementing a search engine like Elasticsearch, or something similar, with Processwire?

Sorry if this seems like an elementary question - I have never implemented a search engine, but one must start somewhere in learning how to do so! Thanks, Kass