In my opinion, a pressure washer after the fall season to clean the deck is perfectly acceptable. I would use a fan tip and no more than 2200 psi. This will get off the grime and mildew. Then that would be a good time to apply your stain or sealer again for the new year, but you do not have to.
$400 for a pressure washer sounds like you might be getting a washer that will not fulfill your needs.
You did not mention if this is gas or electric operated.
Anyway, if I was in the market for a washer for all around the home, I would not get anything under a 2200 psi and gasoline operated.
I would want a selection of tips. the fan tip is the most safest for the deck and the car and the driveway and whatever. Remember, you can back the tip away from things like the car to wash it. Then you can get down to 8 inches from a extremely dirty drive way for that grease area.
Then if you spend more, like $700, for a washer, get one with a good engine. Like Honda or Brigs. Watch the Tuchumseh, and Generack. The engines are the most important part of this particular equipment.