I haven't henna'd, but I keep it in the back of my mind so I try to watch for similar issues.

Many people buy henna from local indian stores.  The henna there is still quality henna, but doesn't have the added chemicals that many of the over the counter type henna's do--and is quite a bit cheaper.  I would definetly do a strand test no matter which you choose.

If your concerned about going that road, remember that it will only cost you $50 for the first application.  After that you will only have roots, and maybe an occasional length application.  I've heard that henna also freezes well, so after that first application, $50 could last you almost a year of hair coloring. 

People on here that have henna'd will probably have more advice.  Good luck, and let us know what you decide.

Take stock!!  That's an excellent price!  I get my henna and indigo from Catherine and I usually spent $50 or alittle more.  I'm still trying to get the deep dark color I want so I'm doing the whole head and length.  At 32 inches there's alot of goop(I mean henna and indigo) to be used!!  Any leftovers I have in the freezer and it does freeze for a long time.