Limit the search to only one content type I'm trying to setup a search for only one content type; I've tried to google it and I saw so many stuff that I don't know which one to test and as I don't want to install too many modules just to test things. I'm not actually even sure that I have to install something else to achieve that.

Do you have any hint or best implementation example please?
These instructions by selvam on KnackForge worked for me. Basically, you can use the MODULE_search_apachesolr_query_prepare hook to add a filter to the query before it's run. 

&lt;?php
function MODULE_search_apachesolr_query_prepare($query) {
  if (/* not correct page */)
    return;
  $compiled_filter = new SolrFilterSubQuery('AND');
  $compiled_filter-&gt;addFilter('bundle', 'TARGET_CONTENT_TYPE');
  $query-&gt;addFilterSubQuery($compiled_filter);
}


I haven't actually tried the hook, but I did use the filter and apply it to a query in a different context (API) and it worked as expected.