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.
For a beginner its really bit high investment i would say. Start with $25 if you are going to choose any other altcoins. But yes its worth to send $100 every two weeks in ETH. It becomes $200 per month. Its really a good investment. ETH is growing up very well in the market assuring its profit to their investors. So better go further.