Im not sure if there is a minimum one needs to invest if pursuing that path. But i was thinking of sticking to my ZEC mine and drop 100$ into buying ETH every two weeks.
it will be better if you invest in ICO which will bring more returns but you setting a time frame doesnt work on cryptocurrency world because price of token you have in mind might be high at anytime so its better to set your buying price then once price of a particular token is at your set price will be sold for you. For ICO i have some lists which i believe they have good return on ivestment