My gameplan for buying ALTs is to diversify short, mid and long-term investments. The minimum investment really depends on you (unless there's an ICO that requires a minimum investment).
I usually categorize long-term investments for cryptocurrencies and ICOs that are having a great project concept, whitepaper and roadmap. Few of my altcoins are now set for long-term run, while other cryptos that I have will be for short and mid-term combined.
It is crucial sometimes to decide what altcoin should you pick to invest and hold for short, mid and long-term.
So what are your short-term projects and what are the "flips" you are trying to make on them?
I think the short-term gains are so hard to find, I mean maybe if you look at when conferences and such are planned to happen but with the crypto space being so volatile I think it is easier betting on the long-term projects imo.