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Check out the RepairPal article on tune-ups: http://repairpal.com/tune-up-what-is-it The cost will depend on what you have done, and you should check your owner's manual to see what Mercury recommends for a car with your mileage. Expect $40 or less for an oil and filter change.
I'm not sure where the plug is. You can probably find that information on the Autozone website. You do need to make sure you get an OBD 1 reader, and not an OBD 2 reader. Cars made after 1996 are OBD 2, and it's a different plug.
Probably not a reason to panic. Fill it up with coolant right away, and see if the light goes off--which it should. The problem you want to watch out for is if the coolant level is low because of a leak. Check under your car and make sure there is never any greenish fluid. If the coolant level drops again soon after you fill it, take your car in and have it checked out, because there is something wrong you'll want to get fixed.
The Check Engine Light (http://repairpal.com/check-engine-light) indicates that there's a problem somewhere in the emissions control system. It's common for a thermostat failure to cause this problem with you car, though the light should not be coming on and going off if the thermostat is dead. Make sure your fuel cap is on correctly, and if the light does not go off for good, take the car in for a diagnosis.
It will run, but you run some risks. There's some more information here: http://repairpal.com/check-engine-light There is some potential to do some damage to your truck if you continue to drive, and that can lead to more repair work, but I can't say for certain. One thing to try immediately--go to an independent shop for a cheaper solution than the dealer. It would be best to fix this if you can find a way to afford it.
While I don't think that it's uncommon for these Nissan's to have emissions problems as they age, you shouldn't be having all of these recurring problems. Have you been taking it back to the same shop over and over again? Sometimes, if something is wrong that is damaging components in the emissions control system, a shop will not diagnose the root cause, and will just change the failed emission control component. The root cause continues, and then kills another parts of the emission control system. My first recommendation would be to find a better shop and see if there is a missing diagnosis.
The emission warning light, or Check Engine Light, can come on for a number of reasons. If there is a problem with the measurements made by any of the many sensors that monitor the emissions of your vehicle, the light will come on. There is at least one trouble code that your engine computer has recorded that is causing that light to come on, and you now should track down a good diagnostic technician who can figure out what the root cause of the problem is. There's more information here: http://repairpal.com/check-engine-light
I have an '01 Pathfinder, too. It's been a great car. The Service Engine Soon light is a Check Engine Light (http://repairpal.com/check-engine-light). There's something amiss in the emission control system, and I recommend taking it to a good diagnostic tech. I've had this problem--it was an evap system issue. Sometimes you can have problems with the ignition coils, too. Not sure about the radiator fan. I'd bet you need to change either the assembly or the motor.
I'd be a little concerned that it has been "chipped". That may mean that it's been driven really hard. Sounds like there's been quite a bit of good maintenance work done, but if you're serious, I'd have a technician inspect it and give you his opinion. Have an expert put his hands on the car and tell you what needs to be done. Buying a car with problems is a little risky, especially one that is this old. If you're looking reliability for this price, you may want to look at late nineties Honda Accords.
I looked into a replacement key for my 2001 325Ci a few years ago, and it was $250 at the dealership. That was for the integrated key and remote--note sure if they had those in 1998, too.