Headsup Poker is one of the toughest games you can play. First of all it depends on what you want to play, if you are playing heads up cash games, get ready for some hardcore swings and build a bankroll that is bigger than your house. We are talking 100-200 max buyins of your limit here at least. As headsup poker can lead to tilt, and lead to it very fast. If you can't stand that, don't play it.
Heads up Sit and Go's on the other hand, don't pose such a big risk from my point of view, as you can tilt here as well, but if you are skilled you can usually grind out a lot more and you can easily drop down in levels. You will always lose sometimes, but it's quite hard to lose 10 games of headsup poker in a row, unless you tilt hardcore. And if you are prone to that, no bankroll management will save you. You should try yoga or meditiation instead. SO with this said, I think that 25 buyins for low level headsup games is more than enough, just make sure you win more than you lose and more than you pay in rake.
A good way to start a headsup journey is to get yourself a free poker bankroll of 50$ and start at the low headsup limits and try to grind this up. You will see pretty quickly if you are a headsup or a shorthanded/fullringe player type.
