Is it worth getting the Xentry? If so, is there a reputable source for the software? There’s a few options on eBay but with a price tag of $1k+, I need some confidence in what I’m buying. Also, dos Autel offer anything similar that can perform the same tasks? Have a 2014 e63s and looking to reset the trans, program the headlights, and simple diagnostics.

There is a banner on the top of the forums you can get xentry from.

maybe I’m not seeing them on the mobile site

talk to Ben’s ninja. Be clear about what you wanna do and what you think you might want coaxed. Coding is not easy on xentry and many people use Creader for the simple coding.

BenzNinja.com
Sure it'll set you back a bit with the hardware and all, but the BenzNinja service is for life and includes any/all MB cars you own, forever.
I don't believe there is anyone who got the service and regretted it. Pretty sure it's 5 stars across the board.
Go to the website and click on the videos, and all the info links, which I think explains it all.

Figure what it costs to go to the dealer, when you have a problem. BenzNinja is basically no different but I bet he's more knowledgeable about your car than anyone at the dealer, and can do things the dealer will not, or cannot, do. I'd imagine it wouldn't take long to rack up the same cost at the dealer, plus with Ninja you can do it at home.

I joined almost four years ago, and while I haven't had much of a need, I am still glad I did it. Mostly I use the software myself, which I imagine most people do. The pkg also includes DTS Monaco and Vediamo, which you can google, and he'll probably use one of those to do the mods you wanted.

When you need him you just msg him to coordinate a time to remote in to your laptop. Your job is to be ready, with the laptop and hardware connected to the car, and good internet connection. You should also have the laptop on charge if you suspect it may die, and same with the car batt because they suck down a lot of juice doing this. My car draws ~15A and I believe yours will do at least that.

Troubleshooting fixes and what not are all free, part of your membership. If you want custom coding then he charges, and think the common coding things and $ is listed on the site? I don't think you'll get it cheaper anywhere else.

Aside from the membership, you need the hardware to connect to the car. I bought a "C4" unit, which I think cost $400, but Ninja will tell you what to get. And you need a computer, ideally a laptop, obviously. I use an Acer Aspire that I carry with me everywhere. I have the BenzNinja software on an SSD that I store inside the laptop in a little ziploc bag, and I just swap it with my normal SSD when I need to use it. Point being you don't need a dedicated laptop for this, just a drive. I think 256G is the min, but I'd go with 512 just in case.

Loading the software took a while to do remotely and occasionally he asks you, via text, to restart it. To start he needs it loaded with a fresh/clean copy of Win 10 (or maybe 11 now?) and no tinkering! Found that out the hard way... So just load Windows and make yourself Admin and give him remote access, which he will explain. I think it took three, maybe four hours because it's a lot of data to Xfer and setup stuff he has to do. I think it took >220G of space altogether? Then you're good. Go plug it into your car and use Xentry to poke around, or have him code whatever you wanted. Considering how much time and work he has to do to set it up, I would make a backup of the drive. Plus I'd imagine he'd charge for a second round. I would. If unfamiliar with backing up a drive I can explain, which is easy imo, and free unless you need an adapter or something.

An Autel might do what you want, this time, but next time it may not. An issue I had with Autel was trying to figure out what all the different models do. I didn't want to spend $800 only discover I needed the $1200 unit, or $2400 unit. I tried contacting Autel, their reps, websites etc and nobody could tell me what the actual difference was between the models. They just say this one does 19 things, that one does 22 things etc. Well, what things??? Can it reload my ECU? Can it code a misc new module? Apparently they don't know... The best answer I got was one rep said "The $1200 model "should" do what you need". So I ended up buying a super cheap Autel AP200 for like $70, just to see, and it was pretty cool and they claim it can do as much as the $800 model, but it's nowhere near what the Ninja setup can do. I haven't used the Autel once since I got Ninja, fyi.
So while the AP was cool for the $ back in 2021, it's not so cool now. Instead you should consider the Launch X431 Elite 2.0 Pro, which is easier to use and faster, but it can also do some preloaded coding, and it's super easy. I bought it mainly to turn off the very annoying Eco Start/Stop function, and as a bonus I made my turn signal only blink once. There are misc other coding things in there but nothing I wanted. Nice tool to have either way, especially if you don't drag the BenzNinja setup around in the car like I do.

So if I could go back in time and tell myself what to get, it would be three things: BenzNinja, the Launch X431, and HP Tuners. I mention HP Tuners because it's very popular with your exact car, and mine, and I'm very glad I have it. Even if you don't use it for tuning, it's an outstanding data logger.

Fyi this is the car charger I bought: Super cheap and works really well. I bought two actually, mainly to have a spare, but when I spend a bunch of time poking around with Xentry I hook both up.

where is it?

Autel will cost you a lot more than $1000 over time. There is an annual subscription in addition to the device you get. XENTRY is the better choice.

https://benzninja.com/mbw/ gets you special pricing.