Hi Jane,
Is immediate reconstruction an option for you? I was a D cup when I had my unilateral mastectomy and they put the tissue expander in at the same time, so I woke up with a 250ml breast shape. It is easy to then pad that out until you are fully expanded.
Even if you can't have immediate reconstruction you should find that about 12 months post mastectomy you can start the reconstruction process so the prosthetics don't need to be a forever thing.
As for playing sport with a prosthesis you could always try something lighter than the traditional ones and use a Knitted Knocker in there, would also be easy to wash when you got all sweaty..
I'd really suggest visiting a sister site Reclaim Your Curves http://www.reclaimyourcurves.org.au/ and having a read about options so you can then go back to your surgeon with specific questions.
Jen