Here is how the car's engine control handles (ECU) the intake temp sensor.
1). The intake air sensor (MAT) is a thermistor. You have the correct. The ECU actually monitors a voltage produced by a flowing current through the thermistor. The ECU will source the thermistor with an excitation voltage that is generally 5 volts. The ECU will have a current limiting resistor built into the circuit.
That being said, I would not use a DAC to try and drive a voltage into the ECU. Use a potentiometer.
The resistance range during normal operation will be from about a 100 ohms to several k ohms. It will NOT be linear. I would use a 1k pot to test the low end and a 10k pot to test the upper end.
Next, Since ground loop problems in an automotive environment are a real problem, all sensor inputs are NOT grounded. You will find that one wire will have the excitation voltage on it and the other will have no voltage on it. Any input you put on the ECU input must be an isolated signal.
Now for testing, If you have access to the car involved, testing could be easy. Generally the intake air temperature is also used for the dash outside air temp. Just set the value of the resistance on one of the pots to a level and attach the pot to the ECU input. Monitor the Dash temperature reading. Change the resistance on the pot and repeat the process. I would start out around a 100 ohms and work my way up. I would expect the high end to be in the neighborhood of about 5k ohms.
If the dash temperature reading does not respond to resistance changes, things get a little bit more difficult. If you have a code reader some cars make the values of all of the sensors available to the code reader. If so, use the code reader to read the value of the intake air temp sensor. It will be labeled as the MAT sensor. If that doesn't work, you will have to do it the hard way.
Take the sensor you have (hoping that is a good one) and expose it to as many different temperature as you can and measure the resistance of the sensor. The problem here is that MAT sensors by nature respond very quickly to temperature changes. So you will have to measure the temperature of the environment and the sensor resistance simultaneously. Here is another problem. You CANNOT get the sensor wet so a water bath won't work. I did my testing using my refrigerator, my freezer and ambient temperature. To get higher temperatures you will need to get creative. I suspended my sensor and reference thermocouple in a covered bottle (not touching the sides of the bottle) and put the bottle in a pot filled with water on my stove. Doing this I could vary the temperature, wait for a stable number and make my readings. It took a while but I was able to go from ambient temperature to about 140 degrees F and get several points. Repeat this several times to you get a good set of repeatable numbers.
Now since the ECU input is a resistance value and it MUST NOT be grounded, generating a signal is a bit involved. They make devises that use an LED/photo transistor pair on a small package. You could probably get them from digikey. The idea here is that resistance of the photo transistor varies inversely proportionally to the current flow through the LED. These devises are optically isolated and work very well. You may have to incorporate an FET transistor to get the range of resistance you need without the PN voltage drop issue but this will work.
Now to generate the curve you need to generate the curve using an algorithm in the Arduino. I would use a slope intercept technique using 5 of more points.
This will be a significant amount of trouble and expense for a 10 dollar item. Have you tried Summit Racing or JEGs? They have a lot of this kind of stuff.
Have fun