CREATE FUNCTION ConvertToBase64
(
    @P1 varchar(100)
)
RETURNS nvarchar(100)
AS
BEGIN
    DECLARE @ResultVar nvarchar(MAX)

    SELECT @ResultVar=CAST(N'' AS XML).value('xs:base64Binary(xs:hexBinary(sql:column("P1")))', 'VARCHAR(MAX)')
  FROM (SELECT CONVERT(VARBINARY(MAX),@P1) [P1]) X

    RETURN...

Xml data type methods are not supported in computed column definitions.
Create a scalar user-defined function to wrap the method invocation.

--DECLARE @myDecimal as decimal(4,1) = 4.5
--DECLARE @myDecimal as decimal(13,4) = 38
DECLARE @myDecimal as decimal(13,4) = 56.78
SELECT RIGHT('00000000000000000' + REPLACE(CONVERT(varchar,@myDecimal),'.',''),13)

An alternative is to use windowed functions:
;WITH l AS
(
    SELECT l.Dealer, l.DescR, l.Attempt, l.Ref, l.Salute,l.Updated_Date, l.Cont_date, l.Compcode,
           ROW_NUMBER()...

There is a problem with the way your script inserts the dates. It just enters them as text so they are not sortable by date, the same applies to the...

"i am seeing if table a has a certain row from certain b"

select *
  from a
 where exists(select *
       ...

Alternatives are:
select * from myTable x
where x.Description like '%text%'
select * from myTable x
where PATINDEX('%text%',x.Description)>0
select * from myTable x
where CHARINDEX('text',x.Description)>0

I doubt...