Torrents are usually not downloaded in order but (for all intents and purposes) randomly. That means that, even though 10% might already be downloaded, it might just be the final 10% of the movie (or more realistically, random snippets scattered throughout the file).
There used to be some torrent clients that were able to do what you ask, request pieces in order only, but I don’t know if they are still around and they could potentially affect your download speed.
No. People who want the benefit of self housing without worrying about hardware will rent a vps or something simpler. The hard part of hardware isn’t the purchase, it’s the maintenance.
Also, why the separate router?