Welcome to the forum.
Downsides - the isolation and the fact that life can lack the sort of every day unpredictability that we take for granted in the rest of the world - though for some, that can be a positive thing. After a long stay, you can become institutionalized with everything that entails. I remember reading one study about people returning from Antarctica saying that they showed traits similar to those of released prisoners!
Who shouldn't go? Well there's various psycholigical profiling that takes place, so the least suitable are sorted out through that, through the references needed and the interview process. If you get homesick easily you shouldn't go, if you aren't pretty self-reliant - mentally more than physically - you shouldn't go, you need to like the great outdoors and you need to like people. Surprisingly maybe, Antarctic bases are far from lonely places, but you do have to make do with what you've got, there's less choice of people and friends. I've been lonlier at home at times than in Antarctica.
How long? well only you can answer that. The winter is a special time and if you don't over winter, you'll probably wish you had. The only people I met in Antarctica who didn't want to winter were those who didn't much want to be there in the first place.