RhythmDoctor: It looks like a decent machine. 3000psi will be sufficient and it looks as if it's a 2.7 gpm flow, also good.
I wouldn't go less than a 25 degree tip, and a 30 or 40 would be a good place to start - and I'd stay away from the rotating tips, they are zero degree (or close to that) nozzles that rotate quickly to cover a larger area. However, even at that, it is still a very concentrated stream of 3,000psi water being directed rapidly over an area.
Distribute the pressure over a fan area with a 25 degree nozzle and then move in only as close as it takes to remove the crud ands not the paint
or finish. Remember, getting too close only once will cause damage, be extra careful.
Work in smaller, controlled areas so you can follow where you have been, and work toward the dirty parts from clean parts, you want the water in front helping to wet the surface you are about to clean to soften the crud you want to remove. Every now and then you can back off and rinse the areas cleaned just to make sure you didn't miss a spot.
You'll find lots of uses for your pressure washer - car washes (great with a bent wand and a broad nozzle 60* or so, for cleaning under the car and removing road salt, etc.), cleaning concrete, etc. but if you want to clean wood be very careful, it gets soft and will scar easily, use a wider nozzle than you think you'll need and maintain caution and distance.