I'm replacing most everything on my Z, and even trying to go as cheap as possible on everything, and using lots of junk yard parts and doing 100% of the labor myself, I still can't find a way to spend less then 15k.
A much better way to do a "full" upgrade, would be to upgrade everything else that isn't engine related first (brakes, suspension, chassis strengthening, seats, paint, bodykit?, etc) before you touch the engine.
As soon as the engine comes out, you'll want to upgrade everything "while you're there". At this point, just add 3-5 years onto whatever time budget you gave yourself unless you're one of those crazy singular focus ultra driven workhorses that apparently has no other hobbies. Man I'm jealous of those people.
There's an Australian guy on youtube right who documented pretty much every aspect of his Z restoration. You might want to give his damn near 90 videos a watch and see if that's what you want to do.
https://www.youtube.com/channel/UCk_vb_SJctymCkwnF6sAwDg