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19" wheels should be avoided. I ordered mine with 19s and when I needed new tires, the only option were the OEM spec all-seasons, the junk OEM Goodyear Wrangler Adventures that came with it, or the OEM Goodyear Duratracs. Next year, BFGoodrich will offer the KO3 in LT255/65R19, so there will be another good option. The 19" tire size is 255/65R19, and the only car on the planet that has that tire size is the Defender.

If you get a P300, you can fit 18" wheels no problem. If you get a P400, just stick with 20" wheels. I put on a set of 20" wheels from an LR4 and got some 265/60R20 Falken Wildpeak AT3Ws. If you really want to, you can grind the rear calipers on a P400 to fit 18" wheels, or you can swap the rear calipers with those from a Discovery 5.

I'd personally recommend sticking with 20" wheels on the P400. Moving to a 265/60R20 gives you a slightly larger diameter that won't rub, and you still have plenty of sidewall. FYI, the maximum diameter tire you can fit on a Defender on air suspension without mods is 32.6." Coil-sprung models have more clearance than an air-sprung Defender in normal height.