While a calculation can be set up to return the model number, it may be much more flexible an approach to set up a related table such that selecting your value in the drop down list looks up the needed data from the related table. That way, future changes to what data might need to appear can be handled just be editing data in the table instead of having to redefine a calculation field.
A possible calculation if you insist on using that method:
Case ( EquipmentType = "First equip type goes here" ; Model number of this type goes here ;
EquipmentType = "Second equip type goes here" ; Model number of second type goes here ;
Model number of third type goes here
Case is a function that you can look up in FileMaker help.
Thanks for the help. I will look that up and give it a try!