Im assuming if you purchased a car with that many miles on it, you checked it out somewhat, and that its running pretty good. I would have a compression test done on it to see if the engine is worn excessively. If its OK, and no oil burning obvious, then just change the plugs, air filter, Oil and filter,fuel filter, and inspect the spark plug wires, cap and rotor and replace as necessary. If you have no idea if the timing belt has been replaced I would spend the money to do that. Its only about 2 1/2 hours labor if I recall right, and the belt isnt to awfull expensive. The problem of ignoring it is that the Sentra does not have a zero interference engine and you can bend the valves and in extreme cases do damage to a piston if the timing belt gives way. ITs worth the money for the security if the car is in otherwise excellent condition. Just an off the wall guess on the labor would be around 200-300 dollars average. I havent done Nissan repair for since 1988 so Im not up to date of course on labor times for timing belt replacement so I could be off somewhat but not a great amount. Its generally recommended to replace the timing belt every 60000 miles. You cant tell most of the time by inspection that it needs it, so you have to go by miles since last replaced.
An Engine rebuild would likely not be worth the cash with that many miles unless you got the car real cheap or given to you. Its an easy chore just to do an incar rering, but anything more would be very time consuming. If the transmission isnt slipping just do a normal transmission service... you can flush all the old fluid out with the right equipment. Check the coolant protection level and replace as necessary, and I would invest in a new thermostat and gasget..as an overheat is almost a guaranteed headgasget job with the aluminum head. Have them pressure test the cooling system and identify any weak hoses or loose clamps. This would be a good start.
