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If your IS doesn't have HIDs, then you wouldn't have to worry about D4S bulbs because your car woudn't have them. In terms of vendors, I would recommend you go with XenonDepot or HIDEXTRA.
I have an HIDExtra kit on both my low beams and fogs. No problems with them whatsoever. Surprisingly I did have issues with my XD kit that I previously had, where the passenger side headlight wouldn't come on every so often. I switched to the HIDExtra kit and problem went away.
As far as going with a 10K kit, that'll really have low light output. I went from a 4300K kit to a 6000K and noticed a dramatic drop in usuable light.