Mexico trademark search and IMPI registration: what should foreign companies know?

One of the first questions foreign companies and startups ask before selling in Mexico: how to run a Mexico trademark search and register the brand with IMPI, the Mexican trademark office, before someone else does.

I would like to collect the experience of the network:

• How long did your registration with IMPI take, from filing to certificate?
• Which Nice classes did you file in, and which ones did you miss?
• Did you face an opposition, or find that someone had already registered your brand in Mexico? How did you solve it?
• Should a company register its trademark before signing with a Mexican distributor?

If you helped a client with this, share what worked, without client details. Answers in English or Spanish are welcome.