Problems with cooling system

My mom has this 2008 Mustang 4.0L V6 with 150k miles on it. I'm thinking about buying it from her but the car has a few issues which no one can seem to quite nail down. I can do some repairs but I'm not an expert by any means. Initially the problem was with the heater. It was taking a really long time for the heater to warm up and sometimes when it was just starting to get warm it would go back to cold. I looked at it and found the coolant to be low and contaminated. The coolant was a brownish red color. I flushed the system out and replaced the thermostat which was bad. For 2 days the hear was great and then one day just as the heat was starting to warm up the temp gauge went all the way into the red for a minute before going back down. Antifreeze was coming out of the coolant reservoir cap. I was suspecting that the pressure in the engine was to high which was the reason but just to be sure I replaced the cap anyways. When I looked at the coolant again it was still contaminated but not as bad. The oil didn't look right either and the computer was generating a P0304 misfire in cylinder 4. At this point I was suspecting a possible bad head gasket. I assumed the misfire was due to fouled out plugs. I took the car to a repair shop and the only thing they found was tons of air in the coolant system. They bled the system and 3 days later the temp was in the red for a minute before coming down to normal. Since then I've done a complete oil change and swapped out all the plugs. The plugs all showed normal wear except for #6 which was cracked. The coolant level hasn't been low in weeks and the heat is fine but the coolant still looks contaminated even after several flushes. The car runs pretty good but I don't want to buy it if something major is wrong with it. The car still seems to miss at idle but seems to run fine during normal driving. The car is still throwing out a P0304 misfire in cylinder 4 and now a P0153 O2 Circuit Slow Respone (Bank 2, Sensor 1). I tested the ignition coil and visually inspected it. I see nothing wrong with the coil. I swapped out the plug wire for cylinder 4 with another and still no change. What do you think the next steps should be?