I strongly recommend you read this thread before buying Elinchrom.
My recommendation would be the B800 with the 30"x60" softbox if you can afford it, even with the USP shipping cost. The 30"x60" softbox can be used horizontally for head shots or for waist up portraits of one person or a small group. Vertically it can be used for standing portraits.
If you can't afford the B800 then check the Flashpoint DG600 from Adorama. Like the B800 it has 6 stops of power control and it has a digital power readout. Note that the power control is NOT calibrated, but it does make it easy to tell how much you have changed the power or to return to a setting you have used before.
Speedrings are specific to the monolight. Most softboxes will fit any speedring.
Be careful mixing different brands of softboxes. Some of the cheap ones don't use proper diffuser fabrics and can cause color & WB shifts to the light. You are best sticking with quality brands and to a single supplier.
For more information I suggest you read my article of equipment for a small home studio.
Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.