Hello there, @Grimayoj.

You can record the purchased parts as an inventory item. And I'll gladly show you how:

1. Go to Gear icon at the top.
2. Choose Products & services.
3. Click the New button at the top and select Inventory.
4. Add a Name, SKU, or Category for what you're tracking. 
5. Select the unit from the Unit* ▼ dropdown.
6. Select the category from the Category ▼ dropdown.
7. Enter the other necessary information.
8. Click Save and close.

Once done, create an expense transaction.

1. In the left menu, click the New (+) icon.
2. Under the category for Vendors, select Expense
3. You will now see the Expense Window. Select a Payee for the expense. 
4. Choose the account the money for this expense came from. 
5. Enter the Payment date and Payment method for this expense.
6. Under Item details, choose the item created. 
7. Enter Description, Amount and other necessary information. 
8. When done, click Save and Close.

If you have completed the equipment parts, you can consider using the bundle feature: Introducing QuickBooks Bundles and Reorder Points. This will allow you to record multiple products or services you sell as a single item.

Don’t hesitate to leave a message below if you need further assistance with your QuickBooks Online account. We’ll be here anytime to help you out. Always take care!