The amount you need for taking a shot at a level is usually much lower than the amount you need to be safe if this is your main game and you have to quit or take a long break if you lose everything. So, you can take a shot with only a few buy-ins to try it out, but this is very different from the amount which will give you a low risk of ruin.

In general, it is easier to lose a tournament buy-in than a cash game buy-in. You might lose a stack every 500 hands in cash games, but you lose a buy-in every time you start playing a SNG, and that might be every 50-100 hands. On the other hand, the mathematical variance is greater when the top prizes are greater. Variance is lower in HU SNGs or DoNs.

The swings you see depend on the mathematical variance and your win rate, and I can't guess your win rate. That is going to depend on your skills including your game selection. Don't worry about what bankroll will make you safe until you establish that you are a winning player.

Any positive ROI can be acceptable. A rough 95% confidence interval for your ROI after n HU tournaments is your observed ROI +- 190%/sqrt(n). For example, if you play 100 HU SNGs with an ROI of 5%, then you have strong evidence that your ROI is 5% +- 19%, or from losing 14% to winning 24%. The amount of play typically needed to be sure you are winning depends on your win rate. If you are crushing low stakes games, then with a little luck you might be confident that you are winning after only 100 games, although you will not know your win rate accurately. If your ROI is 5%, this may be fine (or fantastic in high stakes games), but it will take many more tournaments to be confident that you are a winning player or to pin down your win rate within a factor of 2.

Game selection is important in HU SNGs. For example, on some servers, you have the option to ask for a rematch. If you think you have a good advantage over your opponent, try to make sure he is happy playing you (e.g., by playing quickly, not trash-talking, etc.) and ask for a rematch. Some sites prohibit you from looking up opponents on SharkScope or similar sites while you are logged in, but you can look up players later and see whether you want to play them again.

I think HU SNGs are better for practicing than HU cash games. Many HU cash game opponents quit after a hand or two, so you wait for a long time and then don't get to play much, and you always worry that your opponent will quit. In HU SNGs, you get to play until someone has all of the chips. I addition, the rake is taken per tournament rather than per hand. In low stakes cash games, the rake is very high, and it often means both players lose. You don't need as large of an edge to win in low stakes HU SNGs.