Do NOT pressure wash unless you don't plan on painting for a month or two! If you are hitting your siding with water at 2500 psi it invariably goes in and then has to come out. As well as if you aren't careful, you can ruin your siding. I watched some "college pro" painter destroy the siding on a house.
Preparation is the real key. If your house has peeling paint, scrape everything that is loose and then scrape some more. If the siding has dust or dirt build up, wash it down with a garden hose with a little pressure and wait several days before painting.
I spend my working days painting and have for the last 40 years. I consider myself a full-time scraper and part time painter. Today's paints are "environmentally friendly" but to say it politely, they all suck! Buy a name brand like Sherwin Williams, Benjamin Moore, California, etc. and buy their top-of-the-line paint. Do NOT buy Behr paint or any other less expensive ("cheap") paint. The expense of painting is mostly in labor. The SW "Emerald" line, the BM "Moore Guard/Glo" line or the "2010 Ultra" from California are all very good products and will last the longest. Remember to "chase the shade" and do not paint in the sun.
Good luck and have fun. The one nice thing about painting is at the end of the day you can really see what you have accomplished.