I also own a 2017 Discovery HSE Td6, and in Colorado it seems the Td6 engine sold pretty well. TL;DR, this is my first Land Rover, and I could not be happier with it. The vast majority of people I talk to around here who have them, whether in the Discovery or RR/RRS seem to be quite pleased also.
Mine has been rock solid reliable mechanically for 50k+ miles, and as others have said it does best when it's driven the way diesels should be - they like long miles and they like to be worked. They do not do well in short school runs.
The Td6 engine isn't a Land Rover product. It's a joint Ford-Pugeot development, and is the basis for the PowerStroke that Ford put in the F150 for a few years recently, beefed up for heavier towing appropriate for the PowerStroke badge. It's a strong engine and has been around for a very very long time - since 2014 in the US and longer than that overseas. Be careful looking at forums with lot of British and EU drivers - there is an SD4 engine and those had lots of issues. The 3.0L Lion engines were replaced by the 3.0L 300hp SDV6 twin-turbo (Td6 is a variable-vane single turbo) variant overseas starting in 2019, and LR no longer equips any of their vehicles with these - in EU they've gone to a MHEV Diesel inline-six, the D300, and I'm jealous.
What you'll find is that the vehicle's computer is going to tell you that it needs oil changes way early - sometimes in as few as 4500 miles. This is supposedly an indication of reduced viscosity from fuel dilution. Ignore that. I've tested my oil religiously through Blackstone and the oil is fine - never more than 1.5%, and that was at the very first request - since then, it's been zero or trace fuel in oil. For comparison, the computer supposedly requests an oil change when it calculates dilution is at 6.0%. I change it at 10,000 miles or whenever it's due for a scheduled service (16,000 miles) whether the computer thinks it needs it or not. One thing to note is the the factory-spec Castrol that you buy at LR runs thin to start per independent testing; when I run Mobil1 ESP 5W30, it holds up better, but that's not certified under JLR'S oil spec. My local JLR dealer service center now adds additives with their Castrol to keep TBN up. Take that with whatever caveat you like - if the one you end up buying is out of warranty it doesn't matter, just run a low-SAPS oil that's made for diesels with DPFs. The oil filter is right on top of the engine so it's easy to change without a lift - I use the vacuum method with an 8L hand pumped fluid vacuum pump that connects to the vestigial dipstick tube. Oil changes take less than 10 minutes in my garage.
DEF consumption is a thing. None of the diesels I've had in the last ten years has ever gotten the mileage out of a tank they claim to, so par for the course for me - but at least in the Discovery it's easy to fill and DEF is cheap around here. I get maybe 5,000 miles per DEF tank if I fill it to the brim.
Driving - it is a DREAM. Comfortable, quiet, smooth, and it goes forever. What sound does come through is a nice muscular diesel growl till you get into the upper rev range where it sounds a bit rough. Torque for days, climbs anything I ask it to, up to and above 12,000 feet. If you will tow a trailer offroad, definitely get one with the rear locker. With a rear locker, TR2, and low range, these things are unstoppable. Highway mpg without racks or trailer, I get around 27mpg pretty regularly, and that's in Colorado over mountain passes. I've averaged 26.5mpg from Colorado Springs to Albuquerque, made it on one tank with over 1/4 left when I get there. Not bad for a 4800lb AWD SUV....
Doesn't sound like you'll run it in really cold weather, but running some Diesel Kleen through it once every couple months won't hurt. If I know I'm going to the mountains in winter where it will sit outside in single digits F overnight, I'll put some winter formula in there and it always starts right up.
Take note of the service records - you'll see complaints about infotainment, displays, maybe AC not cooling the 2nd row - the latter doesn't usually apply to ones equipped with 4zone climate, and after 2018 ('19 maybe?) they made improvements like a second fan motor and moving the vents from the back of the B-pillar to the 2nd row center console on the standard 3-zone. In my 2017 as long as we keep the zones synced in summer everyone stays comfy and it cools relatively quickly. You shouldn't see a whole lot of problems with transmission, the engine, or drivetrain in general. If you do, steer clear as it seems those end up being problem cars.
Finally, for aftermarket - check out Lucky8 LLC. You can get Johnson Rods for $100 but IMO they're not necessary - they just trick your EAS to stay inflated, you don't gain any ground clearance, and you lose access height, which can be a problem in some SoCal parking garages, if you need to park in those.
Lots of options for 18in wheels out there, and 33in tires fit without any mods as long as you keep width reasonable - 255/70-18 on the D5 will fit and there are lots of options for good ATs in that size. I still run the OEM 20", 255/60-20 or 275/55-20 both fit with no rubbing.