Deriving sub content types from existing/custom content types

I am looking for the following functionality.

Say I have a custom content type 'Item' with a dropdown box 'weight' (options: kilograms, gallons). I would like to have 2 copies of content type Item - call them 'solid' and 'liquid'. For all nodes of type solid, weight would be kilograms - so I would like to preset the weight field and make it non-editable in the node creation form. Similarly for all nodes of type liquid, weight would be set to gallons. All other fields derived from Item would be set independently for the nodes of types solid and liquid.

I know content copy allows me to copy a cck content type. But I want to capture changes to Item in solid and liquid - copy will not help in this case. In some sense, for my application, liquid and solid should be mere aliases of Item with flexibility to preset some fields (and possibly disable editing on them).

When I search for records in Item, i want the nodes created as solid or liquid to show up. Perhaps, another way of thinking about this application is to design content type Item as a class from which sub content types may be derived.

Is there already some module that implements this? I searched for hours but did not find anything along these lines. If I have missed something, kindly give me a pointer.