sagging cones are difficult, they are metal!
Sagging donuts is your problem. While fitting hilos is a way round it, it's not the best as if they are sagging, then they are knackered. Replacing the donuts will put you back to standard height (although a bit higher while they bed in). If you want to go lower in a controlled manner, then you want hilos.
genuine donuts (not from minisport, rumours are they picked up defect leftovers from the rover closure) will be about £28 each. You'll also want to change the knuckle joints while you're there (about £6 each IIRC). if you tackle it yourself, youll also need a cone compressor. It's not an easy job as they can be complete pigs to get out. Look for the FAQ on fitting hilos for a good write up. (and a good idea of what it takes to change the donuts) Basically, if things go well, it'll take you a day to change all four. Don't scrimp on just changing one side too, always replace them in axle pairs.
With dampers, it's roughly reccomended that if you go about an inch lower, then you need lowered (shorter) dampers to prevent the cone/donut from disldging itself and being bloomi dangerous.
Armed with that info, you need to make a choice of what you want to do, and if you want to get your hands dirty......