FanoftheBlackRifle, getting your C&R is a very good idea. I have mine, and aside from weapons I get a discount from Brownell's and Midway.
If you want to build a rifle many start with a Turk mauser. Many have shot out barrels and are bought for the action. Be aware that there are small ring and large ring Turks. You want a large ring Kale Turk. The large ring receiver can basically shoot anything that will fit in the magazine, so caliber choice is open. There are some issues with some calibers, email me if you want links.
The BATFE has a list on their website with specific weapons that are C&Rs. A weapon is a C&R if it is on the list or is 50 years old or older.
There must be proof it was made prior to 50 years ago though. A letter from the manufacturer works if it is close and in doubt.
If you change a stock or sights it doesn't change the weapon's status as that can be considered a repair. If you rebarrel an action you should log it in your bound book as removed from C&R status. For example, if I buy a Kale Turk, rebarrel it into a .22-250 it is no longer a C&R rifle. If I buy another 98k mauser that has a cracked stock, replacing the stock will not affect it's status. The same applies if I change to diopter sights on my Swede. If it remains in original configuration, not original condition, it remains a C&R.
HTH,
Jim