I can't help you with RAW-video, since I haven't used it that much yet. It's supposed to be pretty good with the 50D, though.
Fixing the exposure settings for video is done by activating Expo. Override and ExpSim in the Expo tab of the settings while using manual mode. When Expo. Override is deactivated, video will have auto exposure, no matter what other settings you use.
The 50D is not a bad camera for video, unless you need sound and even then, synching with an external recorder isn't that hard. High ISO can be a bit of a problem, but even 1600 ISO isn't completely unusable in my opinion. For very dark scenes, I sometimes use FPS override set to 20 and "low light", which gives almost 1/20s exposure, as opposed to the usual 1/60s when using 30FPS and 180° shutter. Sure, it looses the "cinematic" look, but it's better than nothing.