Well, that is just too open ended a question for anyone to give you a real world estimate...
Are you wanting to have it done right ? Do you have any do it yourself urges ... perhaps that you have been fighting to keep in check...? You could learn to fix it yourself, do one area every couple of weeks and prep the car for paint yourself... then spend all the money on the finest paint made... or you can take it to someone else and guess as to whether or not they are really going to do a good job...
Remember, to do a really perfect paint job the primer must rest a week or so after each application of primer... and primer is hydroscopic, meaning that it attracks moisture... all these primered cars you see driving around... are rusting under that primer before they even get the top coat on....
You could have the car completely ready for the top coat and you would know it would last longer than your lifetime... paints are that good now...compared to the old days... like oil technology... amazing strides in the last 50 years.... Since I also have an 81 300 td I did not want you to think we were ignoring you...LOL.. Greg