Any top 11 to 20 alts are worth buying, one or two of these alts can possibly enter the top 10 list sooner or later.
Yes, just always depend on the coinmarketcap.com and the alts that are listed there looks promising to invest but still invest with caution. Though there are some that are newly launched and not listed into that are promising too. But much better if you will first depend to that site which is good to those newbies.
I think you can invest in high volume coins as well, I know they are already appreciated in price but it doesnt mean they wont keep moving upwards, as long as there is an interest for a coin, I think price can only go up. Hence the bitcoin being always up compared to the previous years.
Yes ofcourse you can invest in high volume coins depending on how much you can. Because while taking advantage of the price movement of bitcoin, there would be opposite reaction for the price of alt coins, you wish to invest.