Starting a 1988 buick regal custom( well maintained car that has been idle for 18 months

Besides replacing the battery an doing an oil change, what else should I have ready to start said car. How do I know If its ready for a 500 mile trip? My elderly in-laws kept this up when they were still driving

There is a good chance the battery is discharged, but it might not be beyond saving. If you have electricity available put a charger on the battery for a couple of hours or overnight depending on the type of charger you use. Check the oil, coolant, and other fluid levels to make sure they are OK.

Turn the key on and give it a couple of seconds to see if you hear the fuel pump running to put pressure in the gas lines, then crank the motor and see what happens.

Some cars can sit for a long time, even 18 months and just fire up without any problem.

Drive the car locally a bit and if all seems fine it should be ok for a 500 mile trip. An oil change, cooland drain and refill, perhaps transmission drain and refill wouldn’t hurt. Oil change is the most critical and make sure the proper air pressure is in the tires prior to the trip.

I would do a couple of local driving cycles (get it up to operating temperature and cool down).  If you don't have any indications of a problem at that point it should be good for the trip.  It would be a good idea to have a chase car or a cell phone with you, but I would not really expect a problem.

Since it has been idle for so long, probably low miles and lots of short trips and suspected but unknown maintenance, I’d consider all the engine fluids susupect. Especially since it needs to drive 500 miles before getting properly evaluated. Check the coolant. If you see rust, flush and fill it now. Once home, flush and fill again to try and eliminate and scale build-up in the block and radiator. This is the most ignored fluid in the car, except for brake fluid. Plan on getting the brake fluid changed out as well. It should survive the 500 miles as it is, so it can wait.

Stale gas is another consideration. I hope it is not full. I’d add 5 gals to the tank before start-up from a can of fresh fuel, and once started, stop at the first gas station to fill it the rest of the way. If it is full, that old, stale gas needs to come out before you run it through the engine. Siphon may not work, since a lot of modern cars have anti-siphon baffles. Does anyone have advise on getting 16 gallons of stale gas out of a full gas tank?