The last big improvement in Colonia outfitting was in June. Since then all A-rated modules except the 7A and 8A sensor have been available at least some of the time.
As Rick says, stock levels vary - compare the two:
Ever produced:
https://cdb.sotl.org.uk/outfitting
Currently available:
https://cdb.sotl.org.uk/outfitting/current
...so it may take a few weeks to get a ship to all A-rated, but B- or C- rated will do in the meantime.
as well as more engineers
The Colonia engineers already cover every blueprint in the game (and are easy to unlock), so extra ones would be superfluous. If you want higher grades on those blueprints, then they improve over time as people use them for modules, so if you want e.g. better Fuel Limpet blueprints, head over to Marsha Hicks and engineer some Fuel Limpets.
EDIT: for Arsenic, trading down Polonium and Mercury is probably the most efficient way anyway.
https://www.spansh.co.uk/bodies/search/41926950-0B7B-11EA-8A20-AD2CFF6517A0/1 is a list of the nearest volcanism sites to Colonia which have all three of Arsenic, Mercury and Polonium, which should get you all the Arsenic you need very quickly.