Self-employed computer parts & replacements (monitor, keyboard, HDs, Webcams, etc or any items < $300) where should I enter them in TurboTax Home&Business?

There is a tax election for businesses known as de minimis safe harbor. You will need to make this election so that you don't have to capitalize and depreciate items like this. You can enter your expenses on Schedule C as supplies or computer expense.

Items that cost $2,500 or less can be taken as an expense this year and don’t have to be depreciated over time. To do this, an annual election must be made. It’s called the De Minimis Safe Harbor election.

How do I do this with TurboTax?
After entering your business expenses, you will go to the Assets / Depreciation area. The first screen will ask: Did you buy any items that each cost $2,500 or less in 2018? It’s asking about any items that you haven’t entered yet as expenses.

If you say Yes, here’s the next screen:

We ask a couple of questions and if you say Yes they both apply to you, we ask if you want to take the election to expense items costing $2,500 or less. If you say Yes to that question, TurboTax will add the De Minimis Safe Harbor Election form to your tax return.

The next screen in TurboTax has you review other items you bought. If every item you bought cost $2,500 or less, TurboTax will take you back to Your Business screen and you can enter those additional items in the Business Expenses area and not have to depreciate them.

If some items cost $2,500 or less and some cost more, the next screen asks you about Building Improvements. After answering the questions about Building Improvements, you can determine which items you will still depreciate and which items you choose to expense.