Electrical 99 Ford Taurus

I own a 99 Taurus with 130, 00 miles--I noticed the car was taking a bit longer to start and took it in for a battery check. They checked the 4-year old battery--said it was good, rechecked, said it was bad. Replaced battery and car wouldn't start. Then, replaced starter (hadn't noticed problem before) and car started. When picked up car, remote keyless lock would not work although power locks in car work. Replaced battery in remote entry keychain--still no remote keyless lock. When I drove off, I discovered the front left headlight didn't work. One or two of these could be coincidental--all of it doesn't. Need fix. Thanks.