The lower pressure washer is probably adequate for your needs. However, I prefer the higher pressue. The reason the store wants you to use their detergent is general household detergent and cleaners (supposedly) will damage the internals of the pressure washer. I personally own a 2400 psi washer and it does a great job washing the house. I have NEVER used anything other than the cleaning product recommendded by the manufacturer. It may cost a little more but I'd rather be safe than sorry. As a matter of note... you really have to be careful while pressure washing. The extreme water velocity can damage the paint, wood, siding, etc... You will kind of get a feel for it. Simply move closer or back away as appropriate. I usually apply the detergent with the pressure washer on low pressure. Let it soak in for 3-5 minutes then rinse on high pressure. When using high pressure... if you have lap siding, be careful not to blow high pressure water up under the siding.
Your concern about windows could be well founded. Once again, simply back up a bit and don't hit them full power and you should be OK. I have never had a problem with windows.
One last note.... personal experience dictates... wear a full blown rain suit!!! Including boots. YOU WILL GET VERY, VERY WET!!! Have fun! Mike