Omega Constellation 15-20 years old, value?

[Daneel]

I was in Zurich over Easter and couldn't resist picking up my first gen. It's a Constellation, 18k gold face and bezel (but not case). From post purchase reading, it seems I picked a very ordinary movement, caliber 1111. It has a dark brown leather strap though it's not the original Omega one. It looks lovely though. very classy.

I'm a little concerned I overpaid so was wondering if you guys could suggest what it is worth? Not that I have any intention of selling it, but still.

Also, the power reserve doesn't appear to be great, it's been stopping ~6 hours after I take it off. It has a display back and the movement looks clean and in good order to me but I'm far from an expert. Do I need to have it serviced?