The most expensive suspension mistake is not buying the wrong part. It is buying the right parts in the wrong order and paying for the same labor two or three times. An alignment alone is billable every time you change ride height, and a spring swap on a strut-type front end means the same teardown as a full coilover install.
The order we recommend after well over a thousand suspension installs: dampers and springs together first, then bushings and mounts, then sway bars, then camber and toe adjustment hardware, then wheels and tires last if you are changing offset.
Springs and dampers are a matched system, and this is where most people go wrong. Lowering springs on tired factory struts will feel worse than stock inside six months: the spring rate is higher than the damper was valved for, the shock cannot control rebound, and the car floats over dips and crashes over expansion joints. If you only have the budget for one, wait until you have the budget for both.
Bushings and top mounts are the cheapest handling improvement per dollar on any car past 80,000 miles, and they are nearly free to install while the corner is already apart. Doing them later means paying full teardown labor a second time. Same story for extended studs, camber plates and adjustable top mounts.
Sway bars come after damping because they change the balance of the car, and you want to be tuning balance on a suspension that is already controlling its own motion. Set them last, drive on them, and adjust the endlink position before you conclude you bought the wrong rate.
One more note specific to lowered cars: on most platforms you cannot get a correct alignment below about 1.5 inches of drop without camber and toe adjustment hardware. Budget for it up front rather than discovering it on the alignment rack, and expect to be back for a second alignment after the springs settle at around 500 miles.
