Medium Office is gained after Nuclear Cell and not a moment too soon. It`s boosted selling power will greatly help keeping money flow steady and smooth.
Upgrading to those follows the same rules as switching up earier - do as much as you need to be covered and stay at that.
In spite of much better selling power, heat limits remain the same, that is nigh-negligible. Don`t let it come in contact with heat if you don`t want to replace it.