Is it hard to replace the center link and idler.

The hardest part is removing the old parts. The job will be a little easier if you replace both at the same time since you won't have to separate them where they bolt together. Inner tie rod ends have to be unbolted. Since they will be reattached to the new center link, you have to work the cotter pins out, completely remove the nuts, then dislodge the tapered studs on those inner tie rod ends. The center link also has to be removed from the pitman arm on the steering gear box, and since that tapered stud will also be reattached, the cotter pin and nut must be removed. All of those nuts must be removed first before trying to loosen the tapered stud so that press-fit will hold the studs from turning. That allows you to unscrew the nuts. Many other manufacturers design the parts differently so the tapered studs are a part of the center link and get replaced. You don't have to spend all that time trying to remove rusty cotter pins that like to break off. You can just shear them off with the nuts. On Fords, you have to get those cotter pins out to prevent damaging the threads and so you can stick new ones in.

Once the nuts are removed, you have to "break the taper", meaning get those wedged parts apart. Banging next to them on the link often will shock them apart. There are special tools made that as you tighten a bolt, it pushes on the stud until it pops free. You may be able to borrow one of those from an auto parts store that rents or borrows tools. On your car you can't simply pound on the end of the stud because that will severely damage the threads of the parts you have to reuse. You can also use a "pickle fork" with a hammer or with an air hammer, to force the studs out but you risk damaging the protective rubber boots.

Before you unbolt the idler arm from the frame, measure the distance between the tapered stud end and some reference point such as the ground or a point above it on the car. Make that measurement the same when you bolt on the new arm. That arm needs to be at the same height and angle as the pitman arm so the center link always stays perfectly parallel to the ground. An unlevel link will cause unstable handling.

When you tighten the nuts, first use a torque wrench to set them to the specified tightness. When they don't line up with the cotter pin holes, always tighten them a little more until you can insert that pin. Never loosen the nuts to get the pins in.

The car will need an alignment when you're done. No two center links will ever have exactly the same spacing between the tie rod holes. Just 1/32" difference will lead to noticeable bad tire wear. If you grab the old idler arm before you remove it and force it up and down, you will see the right wheel moves left and right a bunch. If that movement is enough to see by eyeball, you have a LOT of movement that will cause severe tire wear. The slight difference in the orientation of the new idler arm is also reason to have the car aligned.