DC Inductor design for boost converter
My question is what is a good selection of flux density


I wouldn't want to go above a peak flux density of 350 mT at 25 degC falling linearly to 300 mT if the core rises to 100 degC.

My question is what is a good selection of ...specific power density
for this design

Now that depends on a few more things (as well as hysteresis loss implied by core saturation being kept reasonably low). So, work out what H field you will apply to avoid excessive flux density, add gapping if needed (it easily could be needed) and then work out what copper losses you might get with the extra turns (due to gapping) and take it from there.
If it's just core energy density you are considering, this is a guideline from the data sheet: -

But, to transfer this to your design you need to know the volume of your actual ferrite core and this isn't available in the material specification you posted a link to. Note that they are using 200 mT as the peak flux density.