Advice: Just do dollar cost averaging in an index ETF. Question: Why? Answer: This article.
What advantages do index ETFs have over a comparable index mutual fund against the same benchmark? Or, to ask another way, is there any reason to prefer anything other than Vanguard's Index500 vs anything else attempting to replicate the SP 500? (I call out the Vanguard fund because the fees are very low, .14% iirc.)
Mutual funds: recurring investment of any dollar amount (fractional shares). Minimum initial investment required. Trade once per day, beware loads, fees, and expense ratios
ETFs: buy or sell one or more whole shares, trade throughout the day, beware trading commissions and expense ratios.