Is my E400 battery dying or is it just the heat?

[yardleybickford]

I've been driving my E400 around town for the past few weeks and something just feels off with the electrical system. The other morning I went to start the car and it took a good few seconds longer than usual to crank over, and then I noticed the radio presets were all wiped out which was super annoying because I had them perfectly set up. I figured maybe it was just the extreme heat we've been having lately because I know batteries really struggle in this weather, but then yesterday the start-stop system didn't kick in at all during my entire drive which is unusual because it normally activates at every single traffic light. I popped the hood to check the battery and I could see some white residue around the terminals which I think might be corrosion, but honestly I'm not an expert so I don't really know what I'm looking at. I've had this car for about three and a half years now and I don't think the battery has ever been changed, so it's probably well overdue. The dealership is way too far from where I live and work and I honestly don't have the time to sit around waiting for them, so I've been looking for a more convenient option. I was doing some research during my lunch break and came across a place that offers genuine Mercedes-Benz battery replacement near Downtown Dubai, which would be perfect because I work in that area and could easily pop over during a break. I'm hoping it's just a simple battery swap and nothing more complicated because I really don't need another expensive repair right now. Has anyone had similar symptoms with their E-Class and found it was just the battery, or should I be worried about something else like the alternator starting to fail?