Henna is great, but it is also permanent. I suggest you do a lot of research beforehand. Good websites for this are mehandi.com, hennasooq.com, longhaircommunity...etc
1. Henna wont clean your hair like a shampoo. I would suggest applying henna on clean hair. But I suppose when you cowash it out, your hair will be clean.
2. No, Henna is only red. The final color will depend on how long it is left in your hair. The longer you leave it the darker, more burgundy it will become. Shorter times will be more copper-orange. Also henna is only a stain/tint, meaning it adds red tones to the color you already have.
HOWEVER: Cassia obovata is another plant, similar to henna that provides the same strengthening and conditioning properties without the red. Might add golden tones to hair. Results are not as long lasting as henna
3.You can mix your dye-released henna with conditioner, this is called a henna gloss. It is less potent than pure henna.
4. Henna is considered a protein treatment by itself
-Only use body art quality henna from a reputable supplier
-Do your research and strand tests
-Be open minded to the color you get
-Have fun and enjoy your inner hippie