Code readers are OK for engine management lights only. They won't come up with anything unless your management light is on. You'll also have to search google with the error code to get an answer to what it is most of the time.
The cheap eBay Delphi diagnostic kits you can get on eBay are ok for engine management lights, airbags, abs, electronic handbrake caliper rewind and a couple of other bits but won't let you access and modify like vcds.