First Model Newtons, particularly those in good original condition aren't very common, there were around 4000 of them made. The .256 was the most popular, about 2500 of the 4000 were originally chambered in that.
They are fairly valuable in almost any condition, the most valuable ones are in good condition, with matching serial numbers, original sights and butt plate. After that any additional factory options increase their value. Those would include, bolt peep sights or a Lyman 48 reciever sight, barrels longer than 24 inches, raised comb stocks and factory sling swivels.
Modifications that detract from their value are things like, tapping for scopes or stocks shortened and butt plates that were changed.
The stocks are quite delicately shaped and as I said previously, are prone to cracking, so you want to check carefully near the tang.
They were serial numbered in a bunch of places, under the bolt handle, floor plate, grip cap, butt plate, receiver and the stocks tend to be numbered under the action, grip cap and butt plate. Oddly, the barrels seem to be numbered only rarely.
If the rifle is in reasonably good condition and its something you've always wanted, well, I wouldn't let it get away. They are very cool rifles.