I am building a 40X50 metal building to use as a workshop and storage. I have calculated that I need at least 70 amps so am planning on going with a 100A load center. I will us #12and 20 amp breakers. My question is regarding the wiring. I am assuming I should use conduit instead of running romex. I am in a rural area and there are no codes to be concerned about, but I do want to do what is “smart.” If I use conduit can I use plastic or should I go with metal?

UF underground wire can be used. Must be buried at least 18" deep.

Metal corrodes. You can use PVC conduit for the risers, if they need to be protected.

I was asking more about inside of the metal building where I will be running circuits for lights and plug-ins.

Conduit does sound like the right choice. Your choice on materials. Supporting and protecting the romex cables would not be possible.

I think common sense would dictate conduit.

I would use Emt, PVC if corrosive environment.
Like Kiss was saying, If floor wasn't in, buried conduit can save from going a longer way. If you have more than 1 door, I would plan for 3 ways/4 ways?

I would suggest EMT (electrical metallic tubing), also known as thinwall.

Rural, how rural are you? I have a building with a country address 15 miles from the nearest town, still had to pull a permit. A lot of the time the county inspectors don't really inspect but they read the plans. Their main task, I think, is to tell the assessor to rasie you assessment for taxes.