search for text across multiple tables
You have a few different challenges that are affecting performance.


Expressions on the field used to filter, rather than converting the filtered result.
EX: RTRIM( ISNULL( cli.ClientName, '' )) + ' ' + LTRIM( ISNULL( cli.FirstMiddleName, '' )) LIKE '%' + @SearchText + '%'

AND  ISNULL( cli.Flg_Deleted, 0 ) = 0 

Using a scalar valued function in your resultset

RegNum = dbo.Get_B_Car_RegNum( car.ID, GETDATE()),

Using a scalar valued function in a join

ON cli.ID = dbo.Get_B_Car_ID_B_Client( car.ID, GETDATE())

Using a wildcard at the start of the search string

car.Descr LIKE '%' + @SearchText

Possibly, the use of UNION rather than UNION ALL.  


The expressions and wildcards negate the use of any indexes and force a table scan on the underlying table.
As an example changing AND  ISNULL( cli.Flg_Deleted, 0 ) = 0 to AND ( CLI.Flg_Deleted IS NULL OR cli.Flg_Deleted = 0 ) can allow the optimizer to use an index on Flg_Deleted because the desired value is not dependent on an expression

The scalar valued function turns your set based query into a procedural query, (one row at a time) for which SQL Server is not optimized.  See if you can convert this into an inline table-valued function.  You should also consider  dumping the result into a table variable and join on that.  That would at least prevent running the udf for every possible combination of your joined table.

Each UNION will check for duplicates in the top and bottom result sets.  If the data is guaranteed unique between the result sets, you'll gain some performance by switching to UNION ALL, removing the distinct sort across the combined results.