My air handler is over 20 years old and needs replacement and my outside AC unit is fine although it's 15 years old so can I get an air handler that will take R-22 gas now and in the future R-410A?

410a condensers are much larger and need larger evaporator coils in the Furnace. R22 is so expense now, because it is no longer being produced... if you have a leak in the future you will wish you replaced the whole system now. While it might be possible to do make this work, with the aid of a TXV mettering device, I would strongly suggest you replace the entire unit now.