My advice is to try and break down the object into basic shapes and go from there.
A base mesh in my opinion is basic shapes (or primitives, cube, sphere, cylinder, etc. etc.)
My first thought when I saw this would be to sculpt from primitives as @ullreym mentioned.
When starting a model of anything I normally ask myself ‘which primitive object can I use as a base’

Here I added a sphere, rotated 90d on the Y axis so the topology flows towards the cylinder I added next (these are with the default of 32 verts, if it’s too many or too few for your purpose change it to suit your needs in the pop up window when you add the objects, but use the same for the both objects to easily join the two, if mesh modelling and not sculpting, but I would advise make it a habit in case you change your mind later). After this I would delete the rear of the sphere and adjust some verts to get a closer shape to the reference.