Don't buy a cheap flash. At some point, you will want to get a more expensive one.
I don't really have the money and I need one now :/
Actually, I would buy a cheap flash. You can do a lot with manual flash. Look for a used Vivitar 285HV, or even one of the random Chinese flashes, with similar output. You don't get TTL metering control, they recycle pretty slowly, the build quality leaves something to be desired, no auto zoom, or any other fancy crap like that.
But, they have good output, they are cheap, great for off camera flash, lightweight, very portable, and they come in handy all the time. They will work just fine manually with the second curtain setting on your camera. In a club, your exposure is not going to change much, as long as your subject is about the same distance form the camera, from shot to shot.
I like to keep a 285HV, cheap wireless triggers, a super clamp, and a few ball bungee cords handy. This combo is pretty inexpensive, and can be extremely useful.
A good cheap flash is a nice budget way to start, then when you want something nicer, buy it and add to your lighting collection. That cheap flash will still come in handy often as a slave, or second light source, back light/hair light, backup, or as an accent with a snoot. The uses are limitless.