Not really a problem with putting other stuff on it, apart from adhering to security standards. If you want to separate your personal stuff from hosted stuff, go ahead, but just because its torrent, doesnt make it much different.
Put it in a VM if you dont have a second machine i guess.
Dont. They are notoriously bad at such things. Lack of Hardware acceleration mainly. These old Chips and problems with single-board-complications are just not worth it at such high prices.
An Intel N100 MiniPC will have much more compute with less complications.