Have you been to Cancun or Playa del Carmen? They are both pretty awful and expensive. i might piss a few people off by saying this as a Brit, but they were both full of obnoxious rich Americans, actually for that matter obnoxious rich Brits aswell! I would head to Tulum which is beautiful and you can get away with sleeping on the beach there. Then perhaps detour to Cozumel which is spectacular if you can get a cheap ferry ride over, then head inland to Chichen - itza. I didn't get further than that before I had to go back to Mexico City so can't tell you what Merida is like. This is just my opinion of course. if you do head to Cancun or Player Del Carmen and want to eat/can afford to eat in a restaurant or cafe or drink in a bar then head as far back from the beach as you can and it is so much cheaper and more 'Mexican'.
Congratulations on the end of your probation. If I was your side of the Atlantic I would be jumping on this adventure for sure.