looking at 73 240 .. how bad can replacing new frame rails be?

I am about to buy a used 73 240Z in good shape. Has all history and comes with a parts car. It seems that the front frame rails could be rusty. I need to know how hard/expensive it is to have these repaced. Is $ 3000 CAD reasonable for this car/ It has a motor that has been rebuilt proffesionaly a few years ago, mild cam, paint was profesionaly re-done afew years ago and now has some rust showing, and some minor dents and scratches. much of the interior is rough, dash has one crack all guages work well, good rubber and nice mags, reupholstered seats(cloth). Parts car has good motor etc but hit hard in front, factory roll bar.

Both have sun roof in them. does this devalue the car alot? Should I have the roof replaced? The sunroof is a canvas type not a glass type and I really dont likle it much.

I hope someone will reply to this soon as I am thinking of buying this car tommorow.

Thanks alot ... Bruce