I am considering prior art searching (and provided results) based only on non-paid databases (Google, Espacenet, etc), as opposed to paid databases?
Would a careful search in non-paid databases be sufficient (despite being more tedious)?
This is opinion based and I'm an inventor, not a patent attorney. I have done my fair bit of patent searching. In my previous job I've used both Micropatent and Totalpatent both of which are paid services. In addition, I've used the USPTO site, Espacenet, Google Patents and The Lens. I'd say in general the free sites are just fine. The Lens is especially nice in that you can register which allows some additional features like saved searches. Prior art searching is broader than just searching for patents, so don't ignore other useful sites like Google Scholar. I'd like to add the disclaimer that whether a search for prior art is adequate goes beyond the tool being used to search. Professional patent librarians use additional techniques such as classification codes to ferret out prior art that might not use standardized keywords.
The U.S. Patent Office offers several sources of guidance for conducting preliminary patent searches at the U.S. Patent Office databases:
https://www.uspto.gov/video/cbt/ptrcsearching/ https://www.uspto.gov/learning-and-resources/support-centers/patent-and-trademark-resource-centers-ptrc/resources/seven https://www.uspto.gov/sites/default/files/documents/7_Step_US_Patent_Search_Strategy_Guide_2015_rev.pdf
These are quite useful and embody the techniques that professional searchers use. They can also be applied at least in part with searches at the Espacenet database as well.