Since this sub panel is feeding a different part of the same structure as the service, you have to run a 4 wire feeder. You didn't have to buy a main breaker panel, but there is nothing wrong with using it (it just cost you more).
The next question is how many amps do you really need in that box. You can't have more than 100, but you are allowed to have less if you need less. Least inexpensive feeder will probably be 60A using #6 copper wire. If you want the full 100A, the size gets confusing. You can use #3 copper if individual wires if they have at least one H in the type, #2 copper if romex, and #4 SER or individual THHN wires if table 310.15(B)(6) is allowed in your area for this application. Since the subpanel will be close to the main, I'd try to run a 1.25" conduit if possible (PVC or flex metal) and use individual THHN wires.
The 4th grounding wire doesn't have to be as large as the other wires. For a 60A feeder, it only has to be #10 copper. For 61A to 100A, it must be at least #8 copper. No ground rods will be needed for this panel. You will most likely need to buy an accessory grounding bar though, because with the 4 wire feed the neutral and grounding wire must be on separate busses. Also, do not install the green bonding screw into the neutral bus, as that must be electrically isolated form the chassis in this application. On your individual circuits, put the white and bare wires on their respective busses -- they can't share a common bus like in a main panel.