My general rule of thumb is that each panel on a Z costs about $1000 of labour to treat trust / weld in new metal and remove dents etc..
On a Z you have.
Bonnet
Hatch
2x Doors
2x fenders
1 roof skin
2x rear quarters
So that's $9k there...
Then you have the unibody:
Radiator support $1500
Floor pans $1000 each side.
Rails same time...
Rusty wheel well $500?
Sills $500-$1000 P/side.
Accident damage costs more, more pervasive rust that requires extensive fabrication of non available sections, such as firewalls, windscreen pillars etc.. all add extra..
You can see how $20k evaporates fast. I try and do a lot of work on my own car, but rarely satisfied with the level / quality of it. Materials costs + tuition cost a small fortune on its own.
Buy the most rust free example you can.