I will quote Dr Wilson as he is the expert on this:
1)"For therapy, do an enema at least daily, and up to about four times each day. Most everyone can use coffee enemas daily for at least 10 years to detoxify the liver and help heal the entire body."
2) According to Dr Wilson coffee enemas do not cause dehydration, in fact the body absorbs the Coffee-Enema as water and hydrates the body.. "MYTH #2. coffee enemas WASH MINERALS OUT OF THE BODY. No. In fact coffee enemas add minerals to the body. Coffee contains many minerals, including zinc, selenium, potassium and others."
3) The beans Dr Wilson recommends are fresh organic non-oily medium to dark roast. Personally I use a mixture of the medium roast and SA Wilsons light roast as there are benefits to both of these. In a pinch I would use powdered coffee but it won't have the potency of fresh ground coffee.