wow, thats alot of questions
to start, gorwing 20 plants with ur budget probably isnt possible.. you should probably pick up 2-3 1000 watt h.p.s. (high pressure sodiums) with ballasts.. that right there.. if you plan to buy new equipment will be a min. of $450- $600.. then you need AT LEAST 3 inline fans to cool the lights.. another $400 min.. then you need nutes $50-100... soil $50-100 lol so I really cant help you cause its not possible unless you have at least a grand...
Its probably a better idea to start from a seed.. the majority of time seeds are easier to start because they are not subject to disease, bugs, ect.
as far as yield goes.. its dependent on MANY factors.. lighting, genetics, veg. time, ect.