If at all possible, I'd suggest getting the latest model. There's no telling howquickly these will become obsolete, so getting the newest one with the newest processor will ensure that you get at least a couple of years of software support from it. Heck, the original Apple Watch only had about 2 years of support before it was dropped with watchOS 5 which only runs on the 2016 and later models. If you ask me, the Apple Watch is just an expensive iPhone accessory that mirrors what you can do on your phone, but that's just my opinion.

From what I can tell, the Watch lets you receive notifications from your paired iPhone, use apps, make and receive phone calls/messages, perform fitness tracking and other health related tasks, listen to music, etc.

My problem with the device is that it doesn't do anything earth-shatteringly different from my phone. It's not a standalone device. If you don't have an iPhone, it's a paperweight. If it's not constantly paired to an iPhone to transfer data back and forth, it becomes a paperweight. I hear the LTE model gives you a little more freedom, but it's even more expensive than the standard models and still needs to be tethered to an iPhone from time to time. It's not the fantastic wrist mounted computer I dream about, just an overpriced iPhone exclusive accessory.

Having said all that, don't let me persuade you from getting one. If you want a cheaper model, try going for the series 3 without LTE. I'd also recommend trying one before you buy so you can see if it meets your needs. If you do purchase one, make sure you can return it if you don't like the experience. I give Apple kudos for coming out with the first wearable with a screen reader, but the actual device is disappointing. Apple has decided that it's going to be an accessory that will primarily be marketed at people who do fitness and health related things.

@zenothrax nope my phone isn't obsolete, I just made the point that I don't really see the point to get the latest whatever just because its the latest unless it actually does something interesting.

In terms of the apple watch, firstly, what exactly is the difference between the different models? I thought they were mostly like phones, with  basically the same capabilities of the same version other than things like screen size differences, EG all series 3 watches would be largely similar, all series 4 watches similar etc.

As for the capabilities, the things I really wanted were firstly better intigration with a satnav.

and secondly, to get notifications of things and be able to respond when not near my phone, since at the moment unless I'm hovering over my phone I don't always get when say land of livia has a new quest etc.

The Apple Watch is head and shoulders over other watches though. Other talking watches, at least in my experience, tend to crap out after a year or two. Aside from getting coffee spilled on my series 1 by a jerk last year, I haven't had a problem. I would get the series 3, unless you really want those new features and a bigger screen. Granted, it may be better for the 2-finger gestures, but I just don't see the point since the series 4 is so similar to the series 3. Also, from what I read in the manual, the series 4 crown vibrates the watch when it is turned. Um, no thanks. That could get annoying really fast.

The only hardware difference I'm aware of is the LTE model. The LTE allows you to access Internet content and phone calls when you're not tethered to the iPhone. Whether you want to pay the higher price for LTE is up to you. If you have your phone with you, it doesn't matter because the standard model will communicate with your phone via Bluetooth and Wi-Fi.

As for your GPS, I don't think it really helps. None of the blindness GPS apps are available on the watch. You're probably better off buying a compatible Bluetooth GPS receiver.

I bought my Series 4 Apple Watch last month, and I'm really enjoying it. It's much more responsive than my 1st gen Watch. I chose to go with the stainless steel model for the extra durability.
I would definitely say that you should take a look at what is available for the Apple Watch. Make sure that the apps you want to use are available for the Apple Watch.
Apple says that the Series 4 Watch has an S4 processor that is "up to 2x faster than S3". The Series 4 has slightly larger screen sizes, and has ECG functionality built in.