vanguard, fidelity, troweprice are very good mutual fund company. If you only want to buy mutual funds from one of the above companies, then just choose that company as where you put your money. The downside is if you want to buy some mutual funds from other companies, it's going to be very expensive.

As to why vanguard is cheap? Most of vanguard's funds are index funds. It doesn't mean they are bad investment.

scottrade is a broker, so if you want to buy mutual funds from a variety of companies or maybe individual stocks, it's cheaper to try a broker, like etrade, scotrade...