To help you in general:
The customers can use the existing data type users and you can add fields as you need them such as industry they work in, job title, contact info etc. . On the frontend these can be inputted/changed using inputs and buttons and the workflow action “make changes to a thing: “current user””.
Then you have items: so create a data type: item
The data type item can have fields such as price (number) , quantity (number) , condition (text).
You as the admin or users can create new items when clicking a button/input using the workflow action"create a thing"
To easily track quotes sent out, you may also want to create a new data type quote:
Whenever a quote is given, such a “thing” can be created, with the person who asked for the quote (user) , time quote is given (date) , the item (or list of items) in the quote saved whenever such a quote is requested.
This easily lets you list/search past quotes or send/schedule emails for quotes.
It seems like you want to calculate quotes based on the inputs. What kind of calculations are we talking about?
Let me know if you have further questions