Is this easy to do?

As far as I'm aware you can't. Sure there's download managers but I've never found a decent download site!

Jailbroken yep, apps in Cydia for this. Or you can access the web gui of your torrent client through it I guess.

If your iPhone is at home, on the charger, connected to your WiFi then possible. If on mobile data, don't do it - no network allows P2P. If on battery, you will kill your battery.

Do you not have a home PC and broadband connection to do it on?