A new quarterpanel will cost you a good bit since montes are uniside cars. I dont think anyone makes just the quarter panel but I may be wrong. By the looks of that rust, there may be some rust on the inner wheel well as well as the quarter. How good are you with sheetmetal and the welder? That can probably be patched if you can make a good patch panel out of some sheetmetal. I definately wouldnt recommend tackling that by yourself if you havent welded or worked with sheetmetal before. In some cases, a body shop may actually be cheaper since they already have the tools to do the job.

 

When a patch panel is welded in, it's not just layed on top and welded. You cut a hole out of the fender, and cut a filler piece the exact size of the hole, and you butt weld the piece in, so there is no overlap. Done properly, there will be minimal warp, and someone good will fill and smooth it, and you'll never know it's been patched. Will it last? If everything is undercoated good, yes. Even a new panel will rot out if it's not protected properly.