See if you can fire a friend's Mosin Nagant, examine your budget, and see if it still makes sense.
The Mosin Nagant is a clunky and primitive rifle with a funky trigger, sticky action, and almost unusable safety. They have mostly been through at least one World War, and it shows.
You can figure $200 for a Mosin, $75 for the plastic ATI Monte Carlo stock, $50 for bolt handle / scope mount kit. Without a scope and some metal cutting, grinding, drilling, tapping, and maybe welding, at the very least, you are now in $325.
And then it's still a multi-MOA gun with a funky trigger, sticky action, and an almost unusable safety.
Or, for still under $400 still, you could get a brand new sub-MOA Ruger American or American Predator that weighs 2 pounds less with a decent trigger and an actual safety. Or a Savage, or a Mossberg, or a...