Sometimes people have trouble with curl loosening. I have only noticed it for the first day or two following a henna, personally. After that everything is back to normal.
My question for you is, what are you going to do about your roots? Especially if you only want to do it once. I have very dark hair and henna only shows up very nominally. What color are you shooting for? Different hennas stain more red and some stain more orange what final color are you wanting?
Right now I usually use Yemeni (because of it's cherry red tones) or Celebration because it has a high dye content. I have heard that the Celebration supply has run out though. Mehandi usually has the Lawsone content listed, so I suppose that for your purposes the higher the better.
I will give the general warning though. This stuff is NOT coming out without scissors so be really sure and do strand tests!!