I would want the whole car sandblasted - $1000
all rust cut out & new metal welded in, all repaired - $1000
frame PROPERLY repaired - $1000
in and underneath the car entirely rotisserie. - $1000
The whole car stitch welded everywhere - $1000
painted in the color of my choice. $1500 +
cage $1000 + Depepding on how fancy
So your looking $7500 + as a ball park price considering no one can give you a quote for 3/4 of that work until they actually see the car and know what kind of damage they are working with. Secondly, when it comes to repairing frames and such you never know how bad the damage is until you actually start cutting everything away.
But seriously… This little project or idea of yours will cost you a bloody fortune hence why people said you would be better off finding a new chassis.