Kevin,
Although mbrola is not there (raspbian) you can easily install binary from upstream.
Although espeak is there the sound is far from clear if just launch espeak, but using espeak with --stdout option and chaining it to aplay worked for me:
espeak … --stdout | aplay
I'm not sure you'll have a positive experience using Orca while surfing the web (for example) with a Raspberry-Pi. You sould make a lab before buying a hundred :)
And you'd benefit a lot from a bit more processor speed and a bit more memory like those devices chipped with an AllWinner A10 SoC. Search for Raspberry Pi alternatives, adjust your budget and you'll get twice the «power» and «memory».
lnrdo