Have i got this right? NT66U help.

Hi people. Noob in need a bit of help.

Currently i have a Asus NT66U connected to my SR102 as a second router and it is working fine. I would now like to see if i can use the NT66U as my primary router so that i can benefit from the higher transfer rates for HD game streaming and the NAT features of the router. I have read up on so many different threads regarding this and my head seems to be all over the place, so can someone check if i have this right.

1. I will need a BTO modem. Will the Huawei HG612 work and do i need to buy an unlocked one (they seem to be very expensive, well the unlocked ones anyway, is there a cheaper alternative somebody can suggest)?
2. I will need to extract the username and password from the SR102.
3. Flash the NT66U with merlin or DDWRT firmware and enter the username and password into the router settings.

Is anybody using the NT66U in the above way here, is it stable with Sky Fibre? Are the username and password stored on the NT66U or must they be entered each time when the router is booted up? Can the SR102 be used again after this process should i decide to go back to the way it was?

Thank you in advance.