Flying ORD-LIS-MAD on a Discount fare in Nov this year.

Since I did not have the option to pick a seat when booked I am getting emails about paying and booking a seat of choice for a fee of course -

Do I get a random seat at time of checking if I do not purchase one or is it mandatory I will need to pay for a seat no matter what? - Does *A Gold be of any help?

Green vs red is it worth the cost of Comfort seats VS Standard (6ft 4 so assuming leg room is worth the cost)

Thanks!