I have had good luck with 12B4 and noise or microphony. I think that RCA were the most microphonic, and the others about equally risky of being noisy. Most were hard run power supply tubes, so don't take too much stock in that.

If you meant a 750 mV sensitivity, you won't need much gain I think.

On the power TX design, you want enough turns on the primary to yeild a flux density below the knee of the B-H loop. Keep the flux density low and the core loss will be lower. More turns does mean more copper length and thus resistance. There is nothing really magical about TX design. Nothing really complicated either. The output voltage will then be determined by the turns ratio primary to secondary. There is a bit more, and I'll help you find the answers. Not even output TX design is too complicated for a public forum...:)
cheers,
Douglas