I disagree that self-directed (also known as brokerage) IRAs and Roth IRAs are only for people with large start-up capital.
I think they are really best for investors who want more investment options made available than their bank offers, which is usually just a set of funds managed by the bank.
Self-directed IRAs work just the same as regular brokerage accounts but with the tax advantages associated with IRAs. You will pay for trades, but you can invest your IRA contributions in just about any investment vehicle, including mutual funds from your institution or any other mutual fund, bonds and bond funds, ETFs, stocks, and even some types of stock options.
My wife and I both have self-directed brokerage Roths. We enjoy the flexibility they offer in investment choices, and we started them with only $5000 in contributions. In addition to my TSP, I like using the TraderFred supplementals on for ETF recommendations in my Roth.