If your thinking about building from the ground up. Start with a custom action and then work your way down/up... between getting a REM action worked on to getting a custom action... I'd go with a custom action.
.. and then there is the option you could go ahead and just buy a custom rifle that has a custom or trued/blueprinted action.

I think you'll break even going both ways with either buying a custom or building a custom.

either way... have fun