From your post, I expect that you did not get service records with this car. I expect that the transmission fluid and filter was not changed regularly.
Does the transmission work okay? If it works okay, change fluid and filter, drive it and change it again in about 5,000 miles, then go onto the schedule below. If the transmission doesn't work okay, then you must rebuild it. Don't let anyone sell you a new transmission JUST BECAUSE there's crud in the pan of the old one. That could be a scam.
Once you get the transmission replaced drain the fluid from the pan AND the torque converter every 15,000 miles and replace the filter in the AT pan every other time. Do this and your new transmission will last virtually forever.