Listing Inventory items without being purchasable items
You can probably do this with just Drupal core and the Views module if you don't need any e-commerce functionality. It would probably go something like this on a basic level:


Add a new vocabulary called 'Categories' and add your terms to it (engines, pumps, etc.).
Add a new content type called 'Inventory Item'.
Add a term reference field to the 'Inventory Item' content type linking to the 'Categories' vocabulary you just created.
Add whatever other custom fields you need to the content type (price, stock, etc).
Create a view that lists all content of the 'Inventory Item' type.
(optional) Create any other views that you may need to filter the content by whatever criteria is applicable.
Start adding content.
(optional) Grab the Taxonomy Menu module so you can easily create menus that break your inventory items down by category.


The order of those steps is somewhat interchangeable.

If you're not familiar with views there's good documentation out there.