You can just buy an aftermarket anti-glare screen. They are cheap.
You can change the SSD by yourself but you'll lose waranty. There's a noticeable speed difference between the eMMC and NVMe versions. There's no noticeable speed difference between the NVMe versions.
IMHO if can afford it and have a few technical skills buy the little one and install one of these. You might even want to wrap it in one these (there's a real leather option too) or these or replace the joysticks with these (can't get drift and have a much smaller deadzone).
You can change the SSD by yourself but you’ll lose waranty.
Just a FYI, but changing the SSD will not void your warranty. There are two cases you can void your warranty: 1) by opening up your Steam Deck you damage something. 2) you no longer have the original SSD (Keep it safe, because you'll need to put it back when you send it to Valve). Valve have been rather chill about the whole SSD change.
The anti-glare is definitely worth it and it's also very noticeable outside. The other option is that you could just upgrade the screen from ifixit if you're also going to be upgrading your storage anyways.
All versions of the deck allow you to upgrade the SSD. There's actually a lot more vendors selling 2230 nvme drives now. I'd recommend staying away from SABRENT Rocket since their support is nonexistent and they also have a high failure rate. The Corsair MP600 mini and Micron 2400 is a better option.
More than anything I'm sketched out about buying SSDs from untrusted sellers, don't want to pay for 1TB and get 256GB that pretends and gets faulty when filled.
There's tools you can use to verify the disk is right. It's smart to do so before you put it in the deck if you have access to a computer. Just get a cheapest enclosure.
Does the eMMC version allow installing an NVMe drive?
Yes, the only hardware difference between the different version is really only the screen and the drive it comes with. You can technically buy the 64GB version and replace the screen with an anti-glare one at a later date, as it can be bought as a spare part for not much.
Personally I got the 64GB some time ago and am still happy enough with just an SD-card with good speed and decent size. I suspect that will change down the line, but right now I'm happy.
The eMMC one does support installing an NVMe, and from what I've seen the Deck can't really support more than PCIe 3.0 speeds. If you find a good deal on a PCIe 4.0 or 5.0 drive it will still work but there's no reason to spend extra on a newer drive.
I don't think there's a significant power increase from PCIe 3 to PCIe 4. It's just an increase in bandwidth, and SSDs aren't high power devices either.
The anti glare screen is amazing in sunlight. However, if you are like me and have very light coloured eyes, you probably also rely on sunglasses. The anti glare screen will not be compatible with polarized sunglasses. You will not be able to see a single thing.