Agree with Dude here, everyone is going to feel differently about this, so it's up to you mate. I say it's totally worth it and I will continue to sub for an entire year in advance, but you can also use f2p to tip your toes into the water and feel it out first. Whatever the case, welcome back! Cheers
For sure its more then worth the $35. I will say you will want to get the $15 a month membership for late to end game stuff or you wont be able to equip a lot of things required for Heroics+.
Don't rely on us, use the free to play option and see how you like it. If you want to dip your toes in the newest expansion, yes, you'll need to buy the expansion. Honestly, other people can't judge what will make you happy! You have to decide for yourself.