If you are going to assemble a rifle just to assemble a rifle, the Savage action is the easiest to build on.
If you have metalworking skills a Mauser action will serve you just fine.
I don't recommend a Mosin action for a build. I've seen some 45-70 Mosins, and pictures of a 300 WSM Mosin, but they were single shot conversions.
Jimro