What is the name of the coil spring to purchse for Coil Spring for a Ford five hundred AWD

Some autoparts stores lend tools as a courtesy. Things like spring compressors or strut compressors are needed. Some rear struts are made to come off as a unit with the coils made as part of the strut. Others are a 2 piece design and the coil and strut are individually replaced.

A note for the struts is that many need an alignment similar to the common front end alignment after you are done.

Mufflers- these usually come from the factory welded together. You need to allow for the lengths of the pipe and muffler in order to bolt them together after you saw off the old parts. IT HAS BEEN SAID THAT YOU CAN NOT GET A MUFFLER AS CHEAP AS A PROFESSIONAL SHOP CAN. Some are better off re-welded together instead of bolted. You would still have to buy U-bolts but a shop would just weld. Less parts less cost. Also dirt and rusty steel in an eye is not fun.

No one make a Quick strut for that car.

Why? If you want to change the strut, and transfer spring to new strut, some Auto parts stores will let you barrow appropriate tools with a significant deposit. They will probably have instructions with their strut/s.

you have to compress the spring to remove the strut

here in California a spring shop can make them for you from a sample spring. Sacramento area

Have them made at a spring shop.

Just unbolts like any strut

Being a coil over, you need a strut
compression to swap out spring

Automobiles: 2006-2007 Ford Five Hundred 2WD & AWD, Montego

If the steering is getting a little hard to turn at low speeds, this would indicate a loss of Power Steering, and the steering is reverting to "Non-Power" steering, just like the days of the Ford Model T or Model A. Loss of power steering is usually caused by low level of PSF (Power Steering Fluid).
So first on my checklist would be checking the PSF level in the reservoir.
Also bleed (air) from PS (Power Steering) pump.

The Ford Five Hundred has front MacPherson struts, so check these for leakage (of the dampening fluid in the strut cylinder), or breakage of their coil springs.
These springs can actually weaken over time, but this is impossible to see - you must "feel" it by bouncing the strut up & down - preferably with your hands on the top of the strut tower in the engine compartment (rather than on the exterior finish).
If the MacPherson struts have weak/broken springs, these components are not separately serviceable, and the strut would have to be replaced as a unit (and preferably both left/right to maintain smooth/even ride).

The Five Hundred also has rear shock absorbers with springs - check for leaks/breakage/weakness here too.

Steering losing responsiveness/wander - checklist
Priority Action Part Type Cause
1 Inspect Rack & Pinion - Center Wear From Internal Fluid Leaking Around The Rack Piston.
2 Inspect Steering Stabilizer Bracket - Loose or Broken Steering Stabilizer Bracket.
3 Inspect Front Lower Control Arm - Front Lower Control Arm loose/missing bolt(s)
4 Inspect Front Lower Ball Joints - Front Lower Ball Joint(s) Worn/Loose
5 Inspect Front Upper Ball Joints -Front Upper Ball Joint(s) Worn/Loose
6 Inspect Tire Pressure - Tire Pressure should be 35 PSI for the Five Hundred and Montego's P225/55R18 OEM (Original Equipment Manufacturer) tires.

Harsh ride - checklist

1 Inspect Rear Shock/Strut - Shocks/Strut failing/leaking.
2 Inspect Front Shock/Strut - Shocks/Strut failing/leaking.
3 Inspect Coil Spring - Incorrect Coil Spring Selection or Installation.
4 Inspect Coil Spring - Coil Spring broken.

They are stand alone shocks
Here are the part numbers you need.
MOTORCRAFT Part # AST236 Rear Left
MOTORCRAFT Part # AST235 Rear Right
About $100.00 for both parts.

I cheat by using ratchet straps and compressing the springs  or wiring coat hangers around the coil while the vehicles weight is on it or with the wheel off jack the strut up with the weight of the vehicle