Do you have to paint the whole car to repair deep scratch? Please help..?

First, kudos to you for wanting to make things right. It was obviously an accident, but rather than excusing it or shrugging it off, you are really stepping up! Good for you! You should absolutely NOT have to paint the whole car unless the tech painting does not know how to properly match the color- in which case you should walk out of that place and find another. That is the good news. The bad news is to do a good job, you really should have it professionally done and do not try to do this yourself. Depending on what part was scratched will make the repair cost vary, and it can be pretty expensive, but a reputable company will also have a warranty on the paint job. I hope her parents will see what a charm your being and just overlook the whole thing, but if not, again, kudos to you.

1-2k the more you pay the better the paint

You are lucky to be alive!!! ! Most people are VERY protective of their classic cars. Also, that was a VERY stupid thing to do, setting your crap on their car! If the paint is in good condition, just the panel that's scratched can usually be painted. A complete re-paint would be $5,000+. If you've never done any automotive paint work before....DON'T start on their car!. You will only make it worse. Take it to a professional auto-restoration shop & get it done right.