Buying Heavensward immediately unlocks the level cap of 50, even if you can't reach Ishgard yet.

I can't say how much exp you will earn from doing lv.50 dungeons, but the quests still give no exp at 50 (except those added by Heavensward).

If I knew I was going to buy the expansion, I'd probably just buy it when I hit 50. The only exception is if there was a big sale coming up (like Black Friday or Cyber Monday), at which point I'd take a look at my progress and see if I could get by with staying 50 for a while longer. Maybe I'd level some cross-classes or crafters. It's up to you really. But generally, there's a lot more sources of exp now, and the beast tribe quests scale to 50, so later classes you level shouldn't have a problem catching up.