ESR depends on frequency. What frequency did you want to know about?
For most types, DF is fairly stable, until approaching resonance. This can be seen on the impedance plot as resistance falling at the same rate as reactance, but always staying some fraction of it (that ratio being DF).
A wideband model might represent this effect with a diffusion network: a dominant capacitance with R+Cs in parallel, having geometrically spaced R and C values. (Physically speaking, these might correspond to distributed loss elements in the dielectric -- dipoles of random size and relaxation time, which happen to be distributed smoothly enough to give what looks like a constant DF. The actual physics need not bear any relation to this model though -- it's just a model, don't read into it too deeply.)
Many manufacturers have databases including calculators to generate SPICE models; beware that they usually account for only a few of these effects, i.e. the model is still calculated around a frequency of interest.
Tim