If you want to buy the Top20 of all Altcoins with just one trade you should think about investing in Coinoindex:
http://coinoindex.com/Nix uptrend since some weeks.
I don't know. I would like to invest in something like an index, but how could I trust this one?
Poloniex just made a database entry and put it on the nxt asset exchange, where you can buy it, isn't that?
Where is the proof of the actual holdings?
Pffft, don't do it. Everything here turns out to be a scam or incompetently run. If you want to buy altcoins, you're going to be basically just gambling. Like the stock market, it's tough to pick winners. Like the horse races. Like anything in life. Like your wife.