Please take this advice....
Never open a radiator cap on a motor thats been running without first...SQUEEZING the top radiator hose to see if any pressure is in the system. If you cant squeeze the hose in then let it cool down until you can....That will save you from any 3rd degree burns and a trip to the hospital.
A guy used to live in our street who burnt his face off from an erupting radiator because of pressure build up when he opened the cap...now he carrys his scar's for life.
Question: Did you check the fluid levels and condition before you picked the car up ?
The rest I have to say is along the lines of what Fekason said...
Cause if the coolant level was driven around on low for sometime air pockets in the heads may have caused them to crack. (costly fix)
Dirty brown River Murray water is a bad sign the car hasn't been looked after.
What condition / level is the engine oil at ?
Drain it and change it....If it looks like mayonaise....your in serious trouble...engine rebuild nessary as you coolant has leaked into you bottom end through a leaking head gasket or cracked head.
This may explain the burning metal smell...as that would cause engine bearings to overheat and fail.
Signs of a leaking head gasket or cracked head are...
1. excessive water coming out of your exhaust
2. excessive water loss and running hot
3. Water in oil (mayonaise) and visa versa
To check just run your car on the spot without the cap on the radiator warm it up thoroughly until the thermostat opens and water starts circulating through your radiator...if you cant see your coolant top it up until it reaches the neck of the radiator.....if you see constant bubbles in your coolant your in trouble.
regards,
sixshooter