The Rocketeer, 301 and 401 are all considered collectible machines and tend to hold their value really well. There is a lot of information available about them on the internet, and replacements parts can be found for them.
Never heard of Emdecko, so doubt that it is a collectible machine. Don't know much about Necchi other than the old ones were workhorses. I am not aware that they are considered collectible, but they might be. Berninas are very collectible, as are the old Singers, and some people like the old Vikings.
Which machine you choose to buy would depend a lot on condition of each machine, price of each machine, and what personally appeals to you.