I considered to do so. There are a few nice crypto indicies ETFs around but after researching a while, I figured out that it is a very risky investment. You never know whether the coins are still holding by the index creator or not. Therefore it is much better to get and hold your own coins.
I would spread it out among the top 10 or 20 coins by market value. Try to look for bargains and buy up when the price is low. Most coins will fluctuate over time, so you could even make a bit of profit buying when they are low and selling when they are high. By spreading the investment out, there will probably always be ones you can buy/sell on a regular basis.
It is a good strategy. The trend of altcoins is following bitcoin. When bitcoin is up, altcoins are down. Why not invest crytp index?