Really the main thing is do you want to use tungsten or those cheap "big mess of spiral flo" kits?
The cheap kits just don't have the output you'd want for most work.
Long ago I used flat or V-shaped reflectors instead of light boxes. These can be improvised from cheap light stands with fabric draped over a T-bar across the top.
This gives you diffuse light, but isn't a softbox - you can't aim it easily or incrementally, and you have to add a lot of black fabric if you want the spill-control of a softbox. I do this all the time - make essentially an 8' wall of diffuse light with a background stand kit and some grid cloth for instance - but I don't do it when I need a softbox.
Chimera make softboxes to fit different fresnels, but they're really expensive and inefficient (projecting the light onto the front face vs. a bare bulb inside the softbox).
One of the best solutions for a true, tungsten softbox is the Photoflex Starlite line - keep tabs on eBay and you'll find a lot of them, fairly cheap. They use 500w and 1k mogul bulbs.
BUT... how much video are you shooting in tungsten conditions? I use almost all daylight fixtures for video. (I did find a 400w mogul globe with good daylight rendering and use a grow ballast with the starlite, their sockets are pulse-rated... so I have something like a 1500 watt softbox but needs no CTB gels).
Biax fluorescents (kino knockoffs) are the most affordable way to get decent lumens in a soft light; the 4-tube models are under $200, you can clip diffusion across the barn doors for pretty-much an instant softbox, and they're about 1k worth of daylight and pull about 220 watts. LEDs are the hot thing now and will replace all kinds of fixtures eventually - for now they're awfully expensive to get enough lumens to be more than a hairlight.
