It seems that there are two different issues combined in this question:
- Restricting an index entry to a single word, not the complete paragraph;
- Applying different styles in the same paragraph;
To restrict an index entry to a single word, just double-click that word and select (from Menu
Indexes and Tables ->
Entry. In the following dialogue, you can select if the word should appear in the TOC, the alphabetical index, or in a user-defined index.
Applying different styles can be done using character styles for the index entries.
After creating a character style for those entries (in my screenshot: "Indexed Word"), you will have to assign it manually to the words marked as index entries. To navigate between those entries, you can use the navigation window,
index entries as navigation object: