I have a white coupe but the color I’m changing it to is a dark metallic green. Would it help to primer the whole car instead of just the spots with body work? And should I use regular grey 2k primer or should I use a darker grey? Or maybe black?

I would just spot prime your body work areas then with everything done flat smooth ready for top coat shoot a coat of sealer. this will get you to one color for your top coat. you probably could get away without doing the sealer coat as the dark green is probably going to cover any color easy. as for sealer color any color should work if you can't make up your mind use a darker color you'll be ok.
Jay D.

How old is the car?
What condition is the paint in now?

It’s a 96 civic. It’s holding up pretty well except for the hood. The paint is flaking off.

23 year old paint?..... I'd be headed to metal on that hood for sure....rest of the paint can't be too far behind in condition....