The instrument needed will depend on the list of pesticides you are looking for.
https://www.restek.com/en/products/refe ... ure/41952/https://www.restek.com/en/products/refe ... ure/40244/The analyte list in these two standard sets shows the analytes that are common between LC and GC and those that are good on only each different technique. Some analytes are not water soluble and more difficult to analyze by LC, but some are very sensitive to high temperature which will make them more responsive to LC than to GC. Some will respond to both but may give a detection limit difference by a factor of 10x or 100x between the two techniques depending on the structure of the analyte, the ability to ionize by the different methods, ect.
To do everything at the lowest detection limits you eventually need both GC/MSMS and LC/MSMS. What analytes are you most interested in testing, that will allow us to say which would be better to begin with.
As for manufacturer, they all perform similarly, the big difference will be in which software you like the best as they are all somewhat different in how the process the data. My personal preferences are Agilent and Sciex, though I have used Shimadzu for LCMSMS and it was a good third in ease of use for me, others will prefer it over the Agilent though.
For hardware, I prefer the Sciex up to the 6500 models as the sample introduction and analyzer entrance design is simple and easy to keep clean, and you don't really give up much sensitivity versus the Agilent or Shimadzu which have more lenses to clean just inside the analyzer. I don't have any experience with the other manufacturers instruments so can't compare those.