ASUS RT-N16 Wireless Solution For Big House With Thick Walls

I need setting up a wireless home network. I have never done much networking, beyond linking two PCs directly together. This is my first time setting up a network that has multiple PC/devices accessing it.

It's a big house and has two floors. More importantly, it has impossibly thick walls. The walls are between 18 to 24 inches of solid stone (perhaps to repel a tank invasion?). I want to be able to get at least 15-20 Mbps to the furthest reaches of the top floor, in order to stream high quality HD video without a hitch.

I have settled on the ASUS RT-N16 router, unless a better option presents itself. It's one of the more decently priced high-end router available to me. It has decent signal strength (3rd on the MaximumPC router round-up), supports Tomato USB etc. I also intend to use the router to as NAS, plugging in an external HDD to one of the USB ports provided.

Despite its decent wireless strength, I doubt I will get close to the required Wifi throughput. Here is my plan to get around that. Let me know if there are any problems with it or if it can be done better. I intend to run an Ethernet cable from one of the router ports to the top floor, and attach a cheap .n router at the other end, to act as a wireless access point.I am unsure if this will meet all my requirements.

1) Will there be any problems with having one router connect to the other?

2) Will external HDD connected to the RT-N16's USB port, configured as a network share, be accessible as such to any device connecting to the secondary network?

3) What kind of cable will be needed to connect the two routers (provided this configuration is viable)? I remember that cable type used to depend on what's connecting to what, e.g. cross-crimped cable for attaching two computers directly without a hub or switch etc.). I don't know if these concerns are still valid.

4) Will I be able to access the configuration page of the primary router from devices connecting to the secondary router's network? I intent to run some apps on the primary router and would like to be able to check on them from the upper floor as well.

5) Will there be an impact of the speeds? The N16 as Gigabit Ethernet ports, which I doubt the cheap router will have. I don't think that should effect the speeds, but like I said, I have next to no experience in this area.