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5D II is probably the sweet spot for your needs, but also consider older used 1Ds II...better built and better AF than the 5D II, though smaller viewfinder and not as good high ISO. fredmiranda.com, keh.com and adorama.com are all excellent places to look for good used gear.
Following on to the above comments: please, if possible, provide some examples, ideally including relevant EXIF information (ISO, shutter speed, aperture, etc.). There are lots of possible reasons for blurriness but seeing images will narrow the actual reasons down very quickly.
