Need help with a potential loop (or how would you do this)?

Here's something I came up with so far... but I still think there should be a more simple and effective means of doing this without the bulk of code.

SELECT *
	  ,CASE WHEN LEAD([Key], 1) OVER(PARTITION BY [KEY] ORDER BY [Date]) = [Key] THEN 
		    CASE WHEN LEAD(TRANS_CODE, 1) OVER(PARTITION BY [KEY] ORDER BY [Date]) IN ('18', '19') THEN [Amount Paid] + LEAD(Charge, 1) OVER(PARTITION BY [KEY] ORDER BY [Date]) + ISNULL(
				 CASE WHEN LEAD([Key], 2) OVER(PARTITION BY [KEY] ORDER BY [Date]) = [Key] THEN
					  CASE WHEN LEAD(TRANS_CODE, 2) OVER(PARTITION BY [KEY] ORDER BY [Date]) IN ('18', '19') THEN LEAD(Charge, 2) OVER(PARTITION BY [KEY] ORDER BY [Date]) + ISNULL(
						   CASE WHEN LEAD([Key], 3) OVER(PARTITION BY [KEY] ORDER BY [Date]) = [Key] THEN
								CASE WHEN LEAD(TRANS_CODE, 3) OVER(PARTITION BY [KEY] ORDER BY [Date]) IN ('18', '19') THEN LEAD(Charge, 3) OVER(PARTITION BY [KEY] ORDER BY [Date]) + ISNULL(
									 CASE WHEN LEAD([Key], 4) OVER(PARTITION BY [KEY] ORDER BY [Date]) = [Key] THEN
										  CASE WHEN LEAD(TRANS_CODE, 4) OVER(PARTITION BY [KEY] ORDER BY [Date]) IN ('18', '19') THEN LEAD(Charge, 4) OVER(PARTITION BY [KEY] ORDER BY [Date])
											   ELSE 0
										   END
									  END, 0)
									 ELSE 0
								 END
						    END, 0)
						   ELSE 0
					   END
				  END, 0)
				 ELSE 0 END
		    ELSE 0
		END
  FROM #Test
 WHERE [Key] = '93246916'