Apple Airport Express, Extreme and Time Capsules, BT Home Hub, Wireless Extenders confusion

Question

I post quite frequently in Stack Overflow, but use Superuser less frequently. Mainly as I don't change hardware often and rarely have software issues!

I live in a small stone cottage, and have an office in a separate building across a yard.

I have a BT Homehub which is located in the cottage and a series of Ethernet cables running across the yard to the office.

This is fine for my wired stuff.

My main office computers are PCs running Windows 7 Ultimate, and one on Win7 Home, all working fine.

I also have an old laptop on Win XP which works fine wirelessly in the house for those evenings in front of the TV catching up on a bit of work.

I also have an iPhone and an iPad.

Recently, I have been trying to get WiFi in the office so I can use Adobe Shadow (or whatever it now is!) to improve mobile web development efficiency using my iPhone and iPad, so I bought this:

http://www.ebuyer.com/393462-zyxel-wre2205-500mbps-powerline-wireless-n300-range-extender-wre2205-gb0101f

Thinking that would be lovely just plugged into the socket by the door in the office, extending the perimeter of the WiFi from my Homehub. I can't get it to work properly! If I plug a laptop into its ethernet port I can get it to connect to the Homehub and give me a kinda of wired, wireless extender. If, however, I plug the ethernet port into my home hub, it then seems to extend the network, but only my iOs devices work, and all my wired stuff stops working, and seems to create an infinite loop where windows connects to my homehob, and then rather to the internet, it then connects back to the extender thing.

Anyway... in the meantime, I took a fatal trip to the Apple Store, where I purchased an Airport Express... solely for the purpose of hooking my iOs devices up as wireless music players in the house. I knew it had WiFi, but didn't want to use that part as an extender, I didn't think it would work on a Homehub anyway.

It doesn't work on a Homehub!

I now have a new wireless network in the house, which, when anything connects to it cannot connect to the Internet, so it works ONLY as a wireless music player.

I then borrowed some Powerline Adaptors from someone and realised that this whole thing was getting totally out of control!

It seems all the technology is out there but it's so complicated to get the right series of devices.

To further add to the confusion, I wouldn't mind a network hard drive. I bought one that broke and lost everything, so now we're on to looking at the Apple Time Capsules.

So my question is...

IF... I buy an Apple Time Capsule, can I:

1. Hook that up to my Homehub, leaving the homehub connected to the Internet so my Hub phones still work, then disable wireless on the homehub
2. Link up my Airport Express to the Time Capsule PROPERLY so it will connect to the Internet
3. Do the above with an Apple TV box should I buy one in future
4. Use the Time Capsule as a network hard drive to store video and music that can be viewed/listened to via my iOS devices/Apple TV/Aiport Express anywhere even with my main PC off (this currently stores all this data)
5. Hope that the IOS devices like the WiFi from the TimeCapsule better than the Homehub and work without extension, or buy another Airport Express to get WiFI in the office.

Or... should I buy an Airport Extreme and use a USB hard drive for the network drive?