Hi Douglas,
As you probably already know, when it comes to softboxes there are so many choices out there and it can make your head spin.
The good news is that you don't need to agonize over this decision or drive yourself crazy because the reality is that the variations between them isn't nearly as great as you might think.
Softboxes come in a few flavors.... Square and rectangular softboxes, strip softboxes and octagonal shaped softboxes, most often called octa's.
Like any other piece of lighting gear you first must determine your needs and wants, and I see you've listed some of those. Other factors to consider are what you're going to be lighting, the size of the subject area, size of the space you shoot in, and whether it's indoors or out on location.
If you don't know the answers to those questions then there are reasonable solutions that can generically get you through most lighting scenarios, perhaps with some limitations.
Unless you need a particular shape for a specific reason then a rectangular softbox or an octa will do the trick and for all intents and purposes (without getting into a discussion about light), the choice of shape is usually a matter of preference based on the shape of the catch light that appears in the eyes. The other factor for some is based on whether they're creating window light or direct Sun light. Window light will give you a square or rectangle and the Sun will give you a round reflection.
OK, now to your questions.......
These days softboxes are either constructed around a speed ring or built on an umbrella frame. Elinchrom is one of the few companies that has their own proprietary speed ring and a well designed system for quick setup and breakdown. Softboxes built on an umbrella frame are also fast and easy to set up. Standard speed rings can be a bit more of a pain and at times can be downright frustrating but you do get the hang of it or you use a type of speed ring often called a quick ring or quick release ring and those can also make life easier.
There are far too many brands and knockoffs to really make an assessment so here is my bottom line. I use and have used just about every brand available. If you were only basing your decision on the quality of light then you could literally pick any one and it would be great. I've done extensive testing on many light modifiers and determined that most of the differences are in materials used, construction, performance in real world shooting scenarios and longevity.
I know a lot of people that buy no name and Chinese imports on ebay and I've used a lot of that gear myself. For my purposes, quality is very inconsistent and I'm usually unimpressed by build and materials. I find stitching to be crappy and a lot of the diffusion panels are measured and cut inaccurately, adding to displeasure and frustration. I'm not saying that everyone will be sensitive to these things but it is something to consider.
If you're very gung-ho on the umbrella style softbox then honestly, I would just stick with the Paul Buff products. They're not great but they're good, offer a lot of options and are priced for consumers. If, on the other hand, you're fine using other brands then there are many many brands and choices.
One of my personal favorites for excellent quality, beautiful light, easy assembly and disassembly, available grids (may or may not be important to you), customer service and possibility of user replaceable parts is Photoflex. No, there's nothing special or magical about them but I've handled them, I own and use them, recommend them a lot (no I don't work for Photoflex, lol) and know others that have been using the same Photoflex lightboxes for 10 years and they're still goings strong.
As I mentioned earlier, when it comes to efficiency and quality of light there are few variations but I will say that the better quality boxes give you an evenness of coverage across the front panel that the cheaper (lesser quality) boxes don't exhibit. Is it terrible? No. Should it deter you from buying one? Nah.
Price and what we consider affordable is a very subjective thing so that's a tough one. The Buff line of modifiers will work well and Paul offers outstanding service with great pricing so there are good reasons to use his products if you're already using his lights.
There's no reason why you can't get a good quality medium softbox for about $100 or $150. A fabric grid would cost about $100 and that setup gives you a lot of control and possibilities. You could get an octa and that's a great thing but if you plan on adding a grid, be advised that grids for octas tend to be more expensive than the already overpriced rectangular grids.
Hope I've helped. If you have any other questions or just want to discuss, let me know.
If you're interested in one of my tests where I compare a wide variety of light modifiers and demonstrate how the quality of light changes very slightly between some modifiers and with others it's more dramatic, you can see it here:
Comparing Light Modifiers: Part I
