Hi there, nice choice of car.
My MINI came with the armrest and TBH it's not that brilliant compared with ones in other cars I have had. It's not attached to the seat, so if you are not that tall and pull your seat forward then you may find it is only so good for resting your elbow on. The store age is handy though.
I think wind deflector is a must, I very rarely have anyone in the back seats so mine is almost permanently in there.
I swapped the velour mats for fitted MINI rubber ones as they are much more serviceable.
Not really sure about any other extras, mine was more or less fully loaded.