EDIT:
Also instead of an upgrade "slot" type system, I think it would be better to be based on money atleast for the main and required components of the truck, for special equipment I understand the thought process of using slots so that a player cannot create a truck that is capable of all types of calls. Things like pump volume, tank capacity, ect that are required components of the truck should be more expensive to buy for better options but not use a upgrade slot.
basically what i was thinking is you get the vechile what comes with doent take an upgrade slot the apgrade slots are for additional equipment so you want to make a rescue engine so you buy a basic type 1 that comes with 4 upgrade slots it would be like this
Rescue engine
comes with a pump, water tank small, fire fighting equipment basic, than you can add 4 additional things so for a rescue engine you would add
Slot 1-Basic rescue
Slot 2-technical rescue
Slot 3 and 4-vechile rescue
So you get the basic things to classify it as say a engine then you can add additional things to make it a rescue engine or paramedic engine ect