Some questions:

Does TargetObjectType belong in the model?

TargetObjectType of Level N often requires Level N+1.

What types of characterisation are available and necessary?

Full (a posteriori) characterisation is when all values are explicitly given. From thereon one may get more and more vague by giving histograms (representing a probability density distribution), and then various less and less descriptive statistics. For small numbers (1,2,3) of objects one could however fully characterise the experiment already in the metadata using the actual value of objects.

Do we need a datatype in InputParameter?

This is hardly necessary for discovery and moreover can be obtained from the protocol for the experiment. Important are the name, the description+UCD and the value. In the domain model the only thing an InputParameter is is a reference to the Parameter (off the protocol) and the value.