You don't particularly need a Plex server to play recorded videos, there are other options.
But what I really wanted to comment on was that buying a series does not necessarily protct you.
I bought a series from Amazon once, got half-way through and it vanished. Their response was that their contract to stream it had expired and the owner was not going to renew it, even for purchased episodes.
They were quite apologetic, and did refund my money.
I ended up having to "find" the episodes I missed by other means ... As a general principle, if content is easily available, at a fair price, I will pay. When people screw around, make stuff inaccessible, or exorbitantly over priced, I may use other means.