On this link there are listings for the dolly hair equivalent colors for the main release. The site owner may be able to give you the colors for the fuzzy tummy one as well if you were to ask.
http://fairytailbirds.blogspot.com/p/original-12.htmlHow many hanks really depends on how long you want the tail to be. It's about the same as a pony but with different lengths. 1- 1.5 probably. Given that you will likely buy a hank of each of the 3 colors for one then you will have more than enough.
Replacing the hair is super easy. The feet are attached to the body the same way G1 pony heads attach to the body. Both the tail and the top knot are held in similar to G1 pony tails. The body is hollow just like a pony. No real re-rooting just securing.
I have had some pretty rough ones come through my hands as well. Depending on the issues they can clean up really well. Even very tangled hair can be restored with some patients and some helpful products. I would we interested to see their condition.