How to seal molded plastic shower when repainting
The seam between the surround and the wall should be sealed with caulking. You could repair and paint the wall, then use clear caulking at the seam, or repair the wall and seal the seam with paintable caulk then paint. Your choice.

Use a high quality paint and ensure adequate ventilation and no mildewcide is necessary. If you must use it, add it only to the topcoat.