It sounds like your head gasket has blown a leak into the coolant system from the combustion chamber.Strange that the plug is blowing instead of blowing a radiator hose though.Can't be sure on it.
They are used cars 2000 to 2006 Sentra is different than 2007 and 2009. Sentra used to be the base Nissan now Versa takes that place. Every used car needs to be professonally checked out before your money changes hands. Most Sentra are good cars but there are recalls and service items that should be checked out. You might get the seller to pay for if you buy... and you might save a real headache if there is a bad head gasket or cat converter issue.
The ticking sound is caused by the lifters lacking oil. Some engines will shut down as self preservation when the oil pressure falls too low. You have a large oil leak. It could be worn piston rings or the PCV valve is faulty.
Well the early 95 model had the junk v6 engines..The also had alot of recalls so you may want to call your toyota dealer and have them run the vin numbers to see if you are in that catagory...The later 95 models had the new updated v6 that did not have that problem..this was also the first year of the tacoma which had that engine..
smog check is where your exhaust is analyzed to see how many pollutants your car is letting out. the accepable levels vary from state to state and also from metropolitan areas to not metro areas. this is usually required every two years when renewing your registration. (determined by the year of your car- even model year smog check in even years)