MMOs

I hope the following recommendations help you somewhat. ^^

Elder Scrolls Online - The base game is pretty long and you have loads of things to do in it. ^^

I'm pretty sure you can find some cheap keys online for the base game or for any edition you're interested in (CDKeys for example). I usually recommend a Collector's Edition since it usually contains all previous expansions. 

Also it's not necessary to subscribe to ESO Plus, but you gain instant access to the unlimited Crafting bag and all available minor and major DLCs (except the latest released expansion). It's pretty nice if you have money only for the base game.

You can solo most content, except Veteran dungeons (I think).

I recommend using this guide if you wanna play the story in a chronological order.

Star Wars: The Old Republic is another nice MMO, it's one of the best singleplayer online experiences if you're interested only on the story and characters. Plus, it has full voice acting just like ESO.

They recently pushed out an update that changed the class system a bit.

It's a free game, but it has a subscription too which I don't think it's mandatory. 

If you wanna sub though, I recommend playing the game from the official game site. From what I've seen, there are huge issues with the subscription and Cartel coins on the Steam version of the game (something wrong with accounts not linking properly or something).

Lost Ark - I admit, I didn't get to try this one yet (since I play the ones above already), but from what I heard from my coworkers, it's pretty alright. Plus, it's a free game. I don't think it hurts to try it. ^^

And there's also Final Fantasy 14 Online if you like the FF world. ^^
Haven't played much of this, but I think it has quite a lot of stuff to do in it.