As already mentioned, the main thing to look for is frequent oil changes. If you are looking at a 2018 car, then by this time the oil dilution issues were well known, so there really is no excuse for these cars not to have had more regular oil changes. Stay away from cars that have only been serviced at 21k intervals. Again, expect to change the oil at least once between 21k services. Oil changes
can go on the online service history, and are around £150 from a main dealer on cars over 3 years old. The OSH can be checked by anyone with the VIN number online. If you are buying from a dealer, then just look at the VIN on bottom passenger side of the windscreen, and look it up on your phone! Also, if you can pop the bonnet, there's a sticker telling you when the car was actually built, so you can avoid cars that were built early and registered late. Make sure it's built after September 2016 to be a proper 2017 model year car.
I would personally go for HSE Spec, as the SE/SE Tech have woeful halogen headlights and no panoramic roof, this really makes the interior feel more airy, especially if you like the black interiors. However, also bear in mind that HSE spec cars will have been over £40k, and therefore still be costing around £500 in annual road tax, if that's an issue then a 2017 model year HSE would help avoid that and drop to I think £155 for the D180. (If it's under 5 years old, then you pay an extra £355 per year for the first 5 years.) The Landmark models may be an alternative here as they just crept in under £40k (no idea why that happened

) but they won't have any extra options like a tow bar as then they would have been more than the tax threshold. I think the only option you could add was the extra USB ports at about £115. Also you are limited to I think White or Grey and a black interior.
I think looking for pre-facelift cars with the bigger infotainment screen will really limit your choice, it was only available with a very pricey entertainment pack option that included the top spec Meridian sound system as well. Early versions on the first 2017 model year cars were not always upgradeable to have Apple CarPlay and dealers no longer perform the upgrades on these units as it can cause failure. If you are desperate for Apple CarPlay, then there are aftermarket solutions that can do this and keep the existing display and function. I find the SatNav ok on the standard system, but they appear to have stopped updating it now, and charge £90 a time when they do unfortunately.
In terms of maintenance, then what you have to remember is, unlike most of its 'competitors' the DS is a proper 4WD vehicle with all the added complexity that brings, not to mention extra weight. So, it will go through brakes at a higher rate than you may be used to. Tyres I have found last well, or at least the factory fit Pirelli all seasons do, and cost around £125 per tyre on 19 inch wheels. Services are reasonable on the 3+ scheme at a main dealer, but don't forget to factor in the additional oil changes which are vital. Register with My Land Rover online and you will get the option to book direct with dealers at these better prices, some dealers forget when quoting in person. The battery takes a bit of a pounding, so don't be too surprised if it needs replacing around the 6-7 year mark, and I'd highly recommend a CTEK charger to help keep it topped up. I find the D180 runs more smoothly with a well maintained battery. On 2017 onwards models Adblue goes at the rate of around 10L per 6000 miles. (It is half that on 2016 models.)
I don't use the top spec diesel in my car but do use standard Esso fuel and have had zero issues with the DPF over 4 years. I only use supermarket fuel when I have no choice. Also, avoid any high bio fuel content diesel, as bio-diesel has a higher evaporation point than regular diesel making the dilution issue even worse. Not yet an issue in the UK, but be careful if you travel abroad regularly where more bio-content can be found.
Warranties are expensive, but cover most issues. You can pay over 10 months on these with no interest charge, and have the option to maintain Land Rover Assist cover too. They are also fully transferable.
If you buy from a main dealer, I believe you still get a free half day session at a Land Rover Experience centre. Use it!! You'll be amazed at what a DS can actually do.