Take a look at the following two albums:

Move on up - Volume 1 (The very best of Northern Soul)
Move on up - Volume 2 (The very best of Northern Soul)

That should give you a pretty decent guideline of what to put on a NS playlist. As has already been mentioned you won't go far wrong with adding some Tamla Motown in there as well.

I did loads of NS gigs back in the seventies and those albums have a decent selection on them.

Also, might be worth watching this to help you get a feel for it

https://www.youtube.com/watch?v=T_6NS_lLgis

Cheers, and hope the Gig goes well.