A lot of that balance information from TDA is completely useless. Whatever number you have showing for "nonmarginable funds" is the amount of buying power you have. I generally collapse that whole margin information section and never look at it.

You can turn off your margin feature, but it will not affect the timing of the availability of your deposits. Turning off margin also puts other restrictions on trading activities. For example, if you sell stock, you cannot use that money to trade with until the trade clears in 2 days. A margin account has those uncleared funds available immediately.

Make sure you know how both kinds of accounts work so you can make the decision that's right for you.