Guys at the moment I have a F3/5 but want to go into the Linux market with the Nano,only thing is I don't want to fork out for an internal HD at the moment and have a 1TB external USB powered HD, would that do for recording for the time being?
You can use an external HDD, a powered one is better.
PVR HDDs are designed for PVRs, they run cooler and dont need the higher RPM people tend to go for. Any normal HDD will work.
An internal drive is better, the Nano takes a 2.5 one. The caddy is optional. So if you intend to have an internal one, best to buy at same time. Just so you are aware, you can record to a network mount and even share a HDD between receivers.