Forget about the Neewer units unless you are just testing out the idea of studio flash with an eye to upgrading quickly. They are essentially the same units as the Adorama Flashpoint 300WS Budget Studio Flash (there are just some minor cosmetic differences), and while a 3-light kit with stands and modifiers for $300 is a good buy, it's only a good buy for what they are. There is no modifier mount; it uses a friction fit plastic "speed ring" that fits around the nose of the unit. You can get a Bowens-mount adapter, but it just fits over the end of the flash the same way the included softboxes do, so it's not practical to use with heavy (large and/or gridded) softboxes, and definitely not a good idea for overhead use. And since the "standard reflector" is a recessed front dish, it won't be able to fill strip boxes and accessory reflectors (deep parabolics, beauty dishes, and so on) will be essentially useless decorations. Oh, and the power range will leave you with too much power for a lot of shots.

You don't need 400WS units — or even 300WS units, for that matter — for portraiture, but many of the shots you've described are not portraiture. (And $700 for a two-light kit of decent 400WS flashes and accessories is at the bargain end of the scale.) To make something like an office space look like it's lit that way all of the time (yes, I know they aren't, and so does everybody else) you'll need a lot of light and some distance to go with it.

Watt-seconds (or Joules) can only give you a rough approximation of the output, since it's a measure of how much electrical energy the flash is capable of delivering, and not of the light output. But guide numbers are even more meaningless unless you are using a flash with a fixed reflector and no modifiers. Modern large-ish speedlights such as the Nikon SB910 or the Canon 600EX-RT, for instance, will have a guide number of about 60 (metres at ISO 100), but that's only when they're zoomed in to their tightest and pointed directly at the subject. A flash that has a guide number of 48, but at a fixed 28mm setting, may actually put out more total light. For comparison purposes only, the standard output spec for studio flashes is the f-stop needed at ISO 100 at a distance of 2m using whatever the "standard" reflector is for the unit (usually the in the 7- to 8-inch range). For a 300WS monolight, that's usually about f/22 and a bit. So if you are planning to use hard light (rather than a softbox or umbrella), the problem is going to be getting little enough light out of the system for the aperture you want to use much of the time. Conversely, a very large softbox with double diffusion or a large (60" or bigger) soft white/shoot-through umbrella (or a large wall bounce), which you'll need for soft lighting across groups with minimal fall-off, is going to require just about all the power you can throw at the problem (often more than one flash). A more powerful unit that can be turned down very low will come in very handy — the adjustment range is every bit as important as the maximum power output. Most entry-level flashes have a 5-stop range, which will restrict what you can do unless you buy flashes of different power levels (say a 400WS and one or more 200WS units).

Don't get too hung up on any one accessory mount. Speed rings are cheap (well, except for the Profoto mount).

While it is out of your stated budget range by quite a bit (and getting it new means living in North America), the Paul C. Buff Einstein (with all of its foibles, notably the silly Balcar reflector/accessory mount) is an example of the sort of versatility you may need. It has 9 stops of power variability with a 640WS max (that's only 2/3 stops more than 400WS at the same output efficiency at max power, but 3-1/3 stops less at minimum than most 400WS units can provide). It also has a short flash duration; many entry-level units will not allow you to use your camera's maximum sync speed at less than half power, and that will have a huge effect on outdoor use with ambient light. (You will normally have to use a neutral density filter anyway, but long flash means longer shutter speeds, which means stronger ND to rein in the ambient, which means more flash power, which... eventually you'll find a balance between output power, shutter speed and ND filtration, but it will take longer than you expected.)

Frankly, at a budget of $400 or so, you will probably find that one good mid-range TTL speedlight (for the inevitable run-and-gun) and a boatload of cheap manual speedlights (Yongnuos or similar) will cover most of your needs for little outlay. No, they don't have modelling lights, which can make them a little more difficult to work with in a multi-light studio setup. But you have a choice here: time or money. If you can't afford the money, you are going to have to afford the time. (You can always use cheap LED flashlights taped to the flashes to give you some idea of what's going on.) You can get flash brackets that have a Bowens accessory mount, which will allow you to use a variety of inexpensive softboxes and reflectors, as well as umbrellas. And don't be afraid to move your ISO off of 100; it has been a while since ISO 400 was a crime. In the meantime, spend money on stands and grip equipment, and when you're ready to step up to studio flash that will be more of a help than a hindrance, you will know what you need.