Is it worth time to rebuild an old steel frame?
Buying parts alone is sadly a rather expensive way to get a bike and I doubt anyone would be able to tell you if it's worth it to you. That said, I'd be very tempted.

I like old bikes and it's better they get rebuilt and used than end up in landfill.

You should spec up the components you want/need and compare the price with a new one of similar quality and see which you prefer.