if cost of the mining equipment is a major concern, go with your comments below about Asic Antminer L3+ - about $130 each on Ebay.  Don't buy them new, just buy in mass used.  They are not hugely profitable unless power cost is low (and if free, its a big win and ROI would be super fast at those costs per antminer).  For 20 KW you could place about 25 of these, each generates about $600/year right now of litecoin (but if market goes up....).   Thats a 350% plus ROI on the cost of equipment in 1 year, or break even in 3 months.  Doesn't get much better.

More expensive miners (like the Z9 and Z9 minis) look great on paper, but due to people jumping in on it (and its really new) it will have a negative ROI just like the rest of the antminers at some point within the next 6 to 12 months.  And the cost to get those new in demand ASIC (or GPU) would be top dollar, that is hard to earn back .  Buy used on EBAY (or your equivalent) and you can make easy money guaranteed with ROI versus everyone else who has to buy the latest and newest high priced gear to grind out a fast return.  With your free power you are guaranteed a win.

Good luck - tell us how it goes.