I see no reason to go to the trouble of cutting studs and adding headers to make it wider. He says he only wants emergency access. I expect it is probably a hole in between two normally spaced studs. I'd put narrower trim on the wall. Get a piece of 1/2-5/8 MDF and cut a panel to fit the entire hole down to the floor. I'd put a piece of base molding on the bottom to match the rest of the wall. I'd put a piece of the opening trim up about 2 inches above the base. If you do the opening trim pieces w/ 45deg miter cuts you could attach the top and bottom pieces to the door panel and attach the vertical trims to the wall. The ends of the top and bottom trim would overlap onto the wall. This way your door panel could be pushed into the opening and it would stop moving when it is perfectly flush. It might need magnets to hold it or you could glue a piece of foam insulation board to the back of the door panel and make it just a touch too large so it's a tight enough fit to keep the door in place. Paint the base trim white like the rest of the room. Paint the 2 inch section between the base and bottom door trim wall color. Then either paint all the door including it's trims wall color or the trims white. A previous house we lived in had an almost identical access and it was all painted wall color.
I think is is what I'll do. Why leave the 2 inches between base mold and opening trim?
I think my plan is to replicate this same trim, including a base trim which I have an extra section I can use from another project.
I plan to use a piece of drywall or should I use MDF?
painted to match the wall. Put trim on the door and what over hangs onto the wall, I'll put a small weather strip on to seal. and use rigid styrofoam on the back for insulation cut tight to fit the opening and hold it in place.
Or if it doesnt stay in place I will screw it right in place. I dont plan to ever go in the attic space unless I absolutely have to for wiring or maybe a critter (have had bats in there in the past, but fixed that issue already)