First of all, buy a head with adjustable power! :-) Moving heads to control emitted light is usually a bad habit - the closer the soft-box, usually the better the result is! (You want to make the light source to be NOT point-like, so the closer it is, the softer the shadows are, and that's the point.)

Second of all: are you asking how much light you are planning to emit from a soft-box - from us? :-) Your use-cases may greatly vary... You can do portraits with relatively low light, or you can do high-speed-sync action shooting of a group on the field! With a group of heads!

And lastly: it is all different if you are using flashes or continuous light for video... The flash emits a relatively high energy density short pulse, but then the head, the soft-box can cool down during recharge, idle time. If you have continuous light, you need much better ventilation, etc.

The easiest to solve your problem is: find a studio nearby, go there, make some shots, check the properties of the heads and soft-boxes there, and then you will have enough data to purchase equipment. (Make sure though that you go to a place with some good quality equipment.)

My advice: in general, an octa soft-box is always a good idea to start with (creates a light similar to what an umbrella creates, but smoother. Have you tried a simple umbrella and a flash?). I would buy a big octa first, then a huge rectangle soft-box next. I always use these extensively, however, I cannot justify buying them, as I can always go to a studio if they are needed.

Also make sure that you buy equipment for outdoor use if you want to use them outdoors...