Wood siding will need a week to dry, and you'll still have to do some scraping. Of course, it's not for every situation, especially when lead-based paints are involved, but it's often worth considering. Until recently, pressure washers were almost exclusively commercial machines, and they were priced accordingly. In the past 10 years, however, manufacturers have targeted the homeowner with lightweight, general-purpose washers that are priced more like lawnmowers. This is the same unit we used to refinish our deck.
More pressure is certainly useful in some situations, but it's also more dangerous.
All pressure washers can rip through flesh at close range, and more pressure increases the potential hazard. These things may feel like toys, but they're more like chain saws. Better to go slow. Of course, if you're going to need a pressure washer only once a year or so, it makes more sense to rent one. Power washing siding is pretty basic, but there are a few general rules to follow.