[Halcyon]

I think you will probably be fine with the wireless up in the loft as it will broadcast down to the rest of the house.
Netgear products are easy to set up and normally come with helpful wizard set up guides like Linksys products.

What affects your signal strength depends on thickness and the material in your walls, interference, and any other objects restricting the range.
Most connections will be fine with maybe sometimes a few drops in connection but they will connect back again quickly.
You are fine to leave the equipment on all the time, there is no problem with this and most people do leave it connected.

If you did find that you did not get coverage all over the house you could either move the router, or buy an access point to boost the signal, however this would set you back more money.

Try it out. I dont think you will have many problems unless you lived in some sort of castle with lead shielding and metre thick walls.
Good luck.