You say it has a "rock slider", then you say it has a "mud slider" - are these down the sides of the car or under the engine? If its under the engine, that's just a standard part of the car. I'm not sure its its to stop damage to the engine, catch oil drops or just pi$$ owners off when they come to service the cars.
Whether its under the engine or down the side, there's no real indication that the car has been abused off road - unless you can see obvious signs of abuse to it.
The HDC light is probably a fault in the ABS/TC/HDC system rather than the thing being turned on all the time (there's 2 different lights for that anyway) - you need a code reader to find out what part has broke - you have the Teves Mk25 ABS system on that car. When you turn the ignition on, do ABS, TC & HDC lights come on and then the others go out leaving just the HDC? If the other lights don't come on, then the bulbs have been removed and you probably have the 3 Amigos not just 1 Amigo.
Regarding the leak - I believe you just need to find out where its leaking, slow leaks are often difficult to find as the coolant evaporates, with that much leakage, I'd have thought you could see traces of where its coming from.
As said - do not drive a 1.8 unless it has a good coolant levels - the HG can easily go in 10 miles with insufficient pressurised coolant.
Regarding the clunks - as said - various possible reasons inc the ARB bushes, drop links etc - probably one of them rather than damage - but always possible. You are fortunate... its only on 1 corner
Did you know of any of these problems when you bought the car?