The two softboxes for $100 with a CFL bulb inside them won't even touch the sides of the power you need. If you're an experienced event photographer, why can't you just put one or two of your speedlights on stands with a brolly and use that? 90% of the look, 40% of the effort.
Photography
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found some cheap softboxes - 2 for 100$
A softbox is just a light modifier and it has a relatively simple job; softboxes can be made DIY style with household materials and still work fine.
What matters more is the light source you're using inside the softbox. If that's also included in that price, it's probably something like CFL bulbs which are going to be very weak for portrait work.
https://www.reddit.com/r/photography/wiki/buying#wiki_continuous_or_flash.3F
They have a temperature of 6500 kelvin - is this a good solution?
That just tells you about the light color, which you could also freely adjust using your camera's white balance setting, if you're shooting digital instead of film.
Should I invest in more expensive lightboxes where I have more control over the temperature?
You want better light sources with higher output and more control over output.
Color temperature control in the light isn't important unless you need to mix or balance that in a specific way relative to other light sources in the scene The camera's white balance can't help you there because that's a global setting that applies equally to everything in the scene.