Will a 2 PSI natural gas line be sufficient for a tankless water heater?
2 psi!

Where do you live?
Yes 2psi on 1/2" iron can deliver your stateted load a whopping 250 feet!

You can use a 1/2 in regulator down to 7" up to about 15 feet before the tankless. And reduce at the unit.

Also check the tankless for acceptable input pressure, you may not need a reg but most likely you will.

Something like this is totally acceptable not in reference to code