Re: Hello from San Francisco
Welcome to the forum.
I would usually recommend a single speed mountain bike or cruiser with a steel frame for a first build. In the Bay area, you will probably want a multi speed bicycle to help you up the hills. A front brake is a must in the Bay area too (due to hills and moisture).
I wouldn't get choosy on any certain brand of bike. It does need to be in good mechanical shape with true wheels. Yard sale bikes are OK.
I have a 2 stroke, in frame china motor, but rack mount motors and 4 strokes work good too. The four stroke may be better for climbing hills. Good luck.
“You're only young once, but you can be immature as long as you like" -Unknown