I'll make this short and sweet.
I bought FF14 for $20 and played for 2 months back in 2013 when I was sick to death of MoP in WoW and was waiting for WoD to launch (lol). I played the AoE tank class.. I honestly can't remember what it was called....Marauder? I thoroughly enjoyed the time I spent in the game and considered it worth it even just to level. I even upgraded to the CE.
If the leveling experience is as enjoyable as the base game's was, then I'd consider it worth it, even for $40.
My question is this, I stopped doing the main story right when it started requiring raids, how much work would I have ahead of me before I can participate in the expansion if I buy it? I know the expansion requires you to finish the main story in the base game first.