Mq3-45
Grundfos pumps come with a 2-year warranty. Grundfos has a distributor and dealers in Mexico. If you bought your pump in the USA and brought it into Mexico, there will be no warranty, unless you did a proper importation. I am a dealer in Baja and I sell hundreds of these pumps with few problems. When there is a problem, I give my customers a free "loaner" pump to use while I repair their pump. If you live in a humid area of Mexico (it's a big country), there could be problems with condensation in the electronic control box. That has been corrected in their new pumps. There is most certainly a seal on the cover of the electronics compartment. List price on the circuit board is around $260 USD. Who told you it was no good? Was it tested??
If you go with a pressure tank, plan on replacing it every two years or so, and if your water source is not disinfected (chlorine), the bladder will become a bacteria heaven. If you have hard water (over 7 grains), you will need a water softening device or de-scaler before your UV lamp. Read your instructions/specs.
Mexico is a different animal, so you need to ask the right questions. I'd be happy to help you.