I'm not sure which parts are in the GC flex box (maybe a ermitter).
If you want control one device, you need only one ermitter without the 3 port cable.
If you want control more than one device, you need all the parts which you have linked.
Sometimes a new TV comes with a tri-port cable which can connect to RGB input on the TV (it is absolute similar to the tri-port cable for the GC), so take first a look on you equipment, maybe you have this part and can save money.
I hope this info will help you to buy the right parts.