I am also looking to clean and seal my deck this summer. That seems a bit steep to me. While power washing is probably the easiest on the back, there are other ways to do it. One is to scrub it with a bleach solution. My eight year old house has the dryvit system on it.
Water enters the daylight basement at times of wind driven rain.
It appears to me that water may be entering where the ledger board for the 2nd story deck is bolted to the framing. No sign of flashing. Deck is 40' long x 10' wide. I'm guessing water which runs down the outside wall above the deck is following top of ledger back into framing, and appearing in basement where doors or windows keep it from following siding board to the ground. Dryvit is a extremely popular material these days but unfortunately none of the people applying it ever heard of flashing.