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.
ETH is a good investment just like bitcoin and other alts but $100 is quite little for investment and expect a high rate of return unless you try putting them into ICO like VLB. I heard this is a good project and a lot of investors are now looking to it. You can try reading their white paper, at least you have other option for investment that might turn into big earnings.