What does the CTR do that makes it worth $100 with NO buffer or tube (almost twice what a complete M4 stock costs)? Just wondering... never used one. It just looks like an M4 stock with part of it relieved away and the locking lever relocated.
All you need is a flat blade screwdriver to get the old one off (maybe a wrench on the end of the tube of the old one if it is really tight or locktited on) and a CAR stock wrench to tighten up the nut on the new one. That of course assumes you have the stock body, the receiver extension/buffer tube, the buffer, CAR-length spring, locking/castle nut, and locking plate that normally come with a complete collapsible stock. All the parts are different, including the buffer and spring. The carbine buffer and spring are shorter.
You can usually get complete M4 or old-school CAR stocks from the EE for about $50. A CAR wrench will set you back about $5.
The M4-style stocks are usually 6-position stocks, and the tube is a little longer, so they extend out farther. I like the old CAR style (4-position, 3-position, or 2-position, with more positions == better), though. It just looks better to me, and I like the smaller footprint against my shoulder. I'm a tall guy (6' 6") with long arms, and I have no problem using either one. They are both long enough.