If you have to replace some metal around the window it's not that hard to do. You could hand form the pieces out of tin and replace the rusted out areas. Hard to tell you how here, find a good book on metal forming. The narrative and pictures will show you how to do it. Don't be afraid of it, it's just tin! I would say the clown who gave you the $6,000.00 estimate was just being a smarta**. I've got to do a bit of piece making soon on a cab. If you want I'll take some pictures of how to do it and post them for you....