All, I just put a set of 255/65/R17 on our 2017 DS. Working with the installer (Discount Tire) the max recommended outer diameter they could guarantee no rubbing was 30.12in. I don't know if this is recommended by Discount or by LR. I had in mind going with the 245/70R17 which give 30.5in outer diameter. I chose the Falkens instead of the BFGs because of the cost (50USD a corner + full size spare) and I read that the K02s are noisier than the Falkens.
By the way, my list of size calculations and available tires is below as is a picture of the installed tires.
I have not experienced any rubbing or contact in the last two weeks I've had them. The have a bit more road noise and a slight decrease in economy.
Keeping in mind you can probably get away with the 30.5in out diameter without a lift or spacers but you'll also decrease the usable travel and space in the wheel well for chains. (Though I'm not sure where you're going where you would need AT tires + chains.)
to answer your question 17 or 18 there are a lot of good reasons to go with the smaller rim diameter to increase the amount of sidewall especially in situations where you might air down the tires, such as on sand. Larger sidewalls also give greater protection against pinching on hard obstacles. I live in SoCal so sand and rocks were my main concern.
With regards to width you actually gain more contact patch in a air down tire with greater diameter than by going a few cm wider. A wider tire also increases rolling resistance and will decrease economy all other things being equal. However, there is also the look, if you like that. IMO I like the tall narrow cookie cutter look.
You would also have to factor in the expense of getting a 17in rim. I went with the new Boxer from Black Rhino. Personally I think it looks the business.
Next time I need tires I will likely get a 2in lift and put taller tires on. Will you please let me know if you have any questions?