I believe that your copy of the game will automatically allow you to level up to 100, taking you all the way through the content of the previous expansion which was Warlords of Draenor. If it doesn't, then you'd have to buy the WoW digital download from the Blizzard store for £9.99. I am almost certain though that your installation will automatically upgrade to the latest content up to Legion though, which is level 1 - 100. That way, you can experience everything from level 1, or play one of your old characters from whatever level you left them on. If you were to purchase the Legion expansion, you'd get a level 100 boost for one of your characters, taking them into the content for Legion. It all depends on what you'd prefer doing.
Levelling is pretty quick these days, and you can run instances through the Looking For Group tool that will put you into a pool with players from a range of servers so you'd have a good chance of getting into dungeons to level. You can level through PvP as well if you fancy giving that a go. You get XP for almost everything now anyway! If you have a tank class, then speccing to tank and queueing for instances will get you into a dungeon really fast as well. Since you enjoy the grind, you'd probably want to do a mixture of questing and dungeons though! As for the current game, it's a moderate grind but it's very much more accessible to casual players. I only get to play for an hour or two per evening, if that, and I'm always able to get something enjoyable done!