I have opened up a subsidiary office in CDMX with the help of a Mexico based attorney and accountant to handle payroll tax filings and what not.

Attorney - Garcia Barragan Abogados. Helped with initial filings and employment contacts. POA to open bank accounts on our behalf.
CPA equivalent Accountant - Anguiano Contadores. Helps with IMSS, SAT, local taxes, etc.
Staffing - Staffbridge for our first two hires, everyone else came from referals. Note that these agencies helped come up with a compensation package for things that a US person wouldnt think of as standard, ie, "vales"
Banking - Banorte. I have found their English support helpful on days where I dont feel like thinking in Spanish.