Well, it is no little light, but I suggest that old style 'potato masher' style flash be considered.
Since the world you are taking is night, don't worry about automatic sensor capabilites in the flash. With dark pavement, etc. it will almost always be fooled trying to turn black pavement into grey pavement, etc. Amanual unit with full power and half power would in my guess be most useful.
Yes, learn how to guestimate distances, and then interpret guide number into the aperture.
The hassy might be your best friend, for it allows you to synch at almost any shutter speed. Then you can fill flash at dusk, or under street lamp illumination to mix the fill and background ambient light. If you lay on a tripod, then the first exposure for foreground in flash light, and background detials in a longer shutter speed (keep same aperture so DOF is not changed).
In these situations I rely on two meters; the old minolta IIIf for the foreground flash, and another, an old pentax spot meter to read what the exposure under dim ambient lighting will be needed to fill in the shadows.