[Batmobile]

Following. I’m interested to see if there’s a 20” wheel option with good range and comfort. I chose 19” for my M50 but I can change my order.

Those are 868M, OP is asking about 862 wheels range.

 

[Batmobile]

If I order a car with 19” tires, will 20” still fit if I get them later? (someone mentioned a while ago that the structure of the car around the wheel area may be different for 20”)

The 862 wheels are not optional in the USA i4, but can be ordered at your dealer as accessory.

 

[Batmobile]

The 862 20” don’t need the lip?

That depends on the width of the tires. If for instance you take the 868M wheels they have a width of 275mm on the back tires and that means they are wider than the i4 body. And that's illegal in I think most countries. Therefore BMW added some sort op "lip" to solve that problem.
Maybe you can buy that lip as an addition to your 868Ms...,