1) How easy (or hard) will it be to replace the whole works?
2) There must be an input, can I remove the old tank without destroying the input lines?
Yes, but read further.
3) Is it possible that JUST the top of tank is 24 inches, where the actual tank is smaller in diameter.
The tank itself is 18" in diameter.
4) Will I be able to detach the input before I take out the tank?
That is what has to be done, but the input is fastened to the tank from the outside. Keep reading.
5) Will I have to bust up concrete to take out the tank?
Definitely. You have to get down to the inlet pipe and unfasten it. The degree of doing that will depend on what kind of connection was made.
6) Can I unbolt the input from the inside of the tank
Regarding the replacement tank. Recommendations? Can I use a smaller diameter tank or do I have to have the same 24 inch diameter model.
That will be the smallest size you will find for a "proper" ejector tank.
--or-- Am I way over my head and I should just call a plumber [grin]
Probably, but he might suggest just replacing the lid and verifying that all the connections to the lid are tight and sealed.