Help! Buying My First Project Car

I am in talks with a man in Texas that is interested in selling his 1965 Plymouth Barracuda Formula S. He wants me to make an offer on the car, but this is my first car purchase/negotiation and I am a bit concerned with how the legal side of things work.
So, the owner says the car belongs to his sister who lives in Portland, but he is located in Texas. He is attempting to locate the title for the car, but has not found the title yet.

I am concerned about the title being in his sisters name and her not being there to sign the title if he does find it. Do I have to have the title signed?

If the owner can not find the title what do I need to make sure I get from the seller before and after making an offer and acquiring the car?

Since I will be moving the car from Texas to Oklahoma, will this affect the title swap or filing for a lost title ?

All information or tips is greatly appreciated! Thanks in advance.
65 formula S.JPG