The Elinchrom Octagonal Softbox. Ideal for full length fashion or head shots. Soft or hard light, adjustable by adding or subtracting accessories. I bought one some time ago and it developed a split after a couple years of use, but the newer models are more resilient. Plus, the new Rotalux system allow for easy setup. However be aware of apparently discounted versions coming in from Asia. Ask the seller whether or not a case is included! The case is normally supplied but some sellers, especially from India, will not issue a case which means a £30-£40 bill when you buy one independently. The proper case is very important as part of the Elinchrom Rotalux system. Without it you could be shortening the life of your softbox. Also keep in mind that a £40 custom tax bill could also accompany your purchase if bought outside the EU (It happened to another photographer who bought from Asia). As you can see, the cheapest can become the dearest once you’ve added a possible £80.00 to the advertised price! Best from a local UK seller I think.
As for flash head requirements: I found that the Octa performed well with either 300w or 600w head, power doesn’t seem to be a big issue.