Lisa here from the Social Care team at Shopify!
Commonly, for car industry themes they need to have effective filtering so customers can select the type of car, part, model ect. and work their way down the funnel to what they specifically need. Themes that also support a large inventory are important for the car industry selling online.
If you're looking for a free theme, I suggest Supply because of its capacity for filtering. Supply also gives a sidebar menu for each category in addition to the main menu header, offering the option for filtering by tags, vendor. See screenshot here.
There are a range of paid themes that are also great for large inventories. You can install any of these themes in your store to preview before you purchase. Here's how to try a paid theme in your store.
Alternatively, you can always add a filter app that will help you organize your categories and make them more accessible for customers. Check out this filter app here called Turbo. Filter apps can be added to any theme that you choose to use, browse more apps here.
When you get started with Shopify, you'll also need to configure all your products into their respective collections regardless of what theme you're using. If you have many products that fit into multiple collections, for example, BMW > 2018 > Radiator, each product would need to be added to 3 collections if you wanted the product to show up under each collection. Here's some more information on how to set up product collections. For your situation with car parts, I suggest you set up automated collections, that use tags to filter and populate the collections. This way you can easily edit and add the products to different collections, and allow customers to search and filter by the use of tags.
Are you currently selling online? Also, do you know what channel (Facebook, Instagram, SEO, email marketing, offline marketing) you expect to bring in the most customers and visitors for you?