The best time to plant is after the last frost. IF the soil can retain water pretty well then you shouldn't have to water. When you guerilla grow you shouldn't visit it more than once a month but most people do anyways so be careful. As far as soil goes... I will bring about 10 gallons (35 liters) of composted horse manure and mix it into the native soil. I plant around blackberry and poison ivy and anything that would discourage visitors.
As far as seeds go you are right next to amsterdam so look at some of the online seed banks and see what they rate as being a good outdoor strain.

http://www.seriousseeds.com/england/start.htm
One of my favorites... kalimist.. Sativa