maybe check in Playa Del Carmen first. The budget places on the hotel zone are usually on the lagoon. Tripadvisor.com might help but that bunch is really focused on the BIG resorts and what to pack, what to drink.....
Check interlinetravel.com for all-inclusives Riveiera Maya and/or Cancun (unless you want to club hop?) Bet you can get a cheap deal now and not pay the fee (like some other agencies) ARe you traveling alone? that might mean the all-inclusives are a poor idea (sorry--just considered that)
You'll need to use collectivos for your budget, but thats likely not a problem for you, coming from Lima?
HAPPY EARLY BDAY!! I remember 30...I think ?