He should build it in Florida. But realistically he’s not going to build it at all. We are no where near ready to go to Mars.
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Even if it never goes to Mars it would be an extremely valuable rocket because of very low cost per kilogram.
The prototypes are being built and tested in Texas already.
I have an idea to get there. Here it is. Send the cargo up in separate rockets and send it on a long journey around the sun several times to use the suns gravity as an assist. Then when it is accelerated to a very high speed you send up the astronauts and rendezvous with it. It takes a lot less energy to accelerate a few astronauts than it does to accelerate large amounts of cargo. Of course you’d have to carefully plan so that by the time it reached Mars they would not have to decelerate much. And the Earth and Mars would need to be in correct alignment.