You can use the power washer even if you don't plan to paint or stain. Be careful though. If the paint is peeling, you will take it off with the power washer and then you will need to prime and paint/stain.
Most people use their machines infrequently and probably don't need an $800.00 piece of equipment. For a really good deal, look up your local power washer dealers and see if they have any refurbished units in their inventory. I paid $200.00 for a really nice refurbished Karcher 4400 G that works great for my purposes.