Advanced search-engine with intelligent listing and synonyms

For a SiteCore 9 site, we need an advanced search-engine.

 

Firstly, current out-of-the-box solution does not offer any capabilities in regards of synonyms. As fx: Spelling-errors or setting up synonyms such as (in Danish) TAXI is the same as TAXA. So, if you search for taxI or taxA, if you get 2 different results.

 

Secondly, we have a list of FAQ's and sub-page-links. I would like to have these intelligently listed, based on popularity. Is this an option?

 

Thurdly, we need page/ content limited search apabilities (search only within this subject/ area).

 

Do you know of any proven search-machines for these matters?