[MagicRat]

Most of these problems are not too complex and are typical of a car which has been neglected by the previous owner. Cheap used cars typically need repair work like this.

My suggestions in the order you have posted.

1. Scan for codes stored on the ECM. There are websites that can tell you what the codes mean, but sometimes these explanations need interpretation to come up with a remedy.

2. Why do you think the thermostat does not work? Usually if they are defective, the engine never fully warms up; the heater does not push out much warm air and the temperature gause does not move much.
Thermostats are easily replaced and are pretty cheap.

If you change one, consider draining out the old coolant, flushing the cooling system and refilling with new, properly mixed coolant at the same time. Coolant does not last forever, if the cooling system has been neglected, its time for a change.

BTW the thermostat has nothing to do with a coolant level sensor.

3. You are correct, that sounds like a blower motor resistor. Usually the resistor pack is still good, but the thermal fuse has blown. If you are handy, you can find the thermal fuse on the resistor pack, and solder in up another one sourced from Radio Shack. However, most mechanics do not bother to do this, they just replace the entire resistor pack.

4. AC systems are best checked out by a qualified mechanic when the weather warms up

5. You mean the radiator fan? If the thermostat is stuck open the fan probably does not need to come on at all and it never comes on unless the ignition is on. The fan relay may be defective or shorted out.

6. In what way is the fuel gauge off? If it reads a higher or lower fuel level than is actually there, the sender unit in the tank may be bad. If the gauge is pinned all the way to E or F, and reads nothing in between there is probably a wiring problem.

7 and 8. The poor mileage and bad smell may be related. If the oxygen sensors are bad, or there is an ignition problem, the fuel mixture may be too rich and/or unburned fuel is going into the exhaust system. Both issues will cause poor gas mileage and will overheat the catalytic converter, creating a bad smell.
Note this condition may damage a converter so much it needs replacement. It also might be a fire hazard.

Also, this issue may be related to #1, shown above.

9. the bad noise sounds like bad bearings in one of the accessory devices, like the alternator, AC compressor, power steering pump or water pump. Sometimes you need to have someone start the car while you listen to the engine carefully under the hood to pinpoint the item which is bad.

#1,2,7,8,9 should be fixed first.