Return Most Recent Entry But Sum One Field
You can use a combination of ROW_NUMBER and SUM() OVER():

WITH CTE AS
(
    SELECT  *,
            RN = ROW_NUMBER() OVER( PARTITION BY custid
                                    ORDER BY rdsdate),
            total_amt = SUM(amt) OVER(PARTITION BY custid)
    FROM @Helper1
)
SELECT  name,
        inputid,
        custid,
        rdsdate,
        total_amt
FROM CTE
WHERE RN = 1;