No, you're not going too far at all
. I still shiver at the "free electricity" part... somebody's got to pay the bill for it, and if you're suddenly increasing the demand by 100kW it'll be noticed. Unless of course by "free" you simply meant, "power costs are pretty much negligible at $0.01 or less per kWh."
While the S3s are good little miners, I wouldn't be considering them for the space you've got. I think you'll be far better served by the rack mountable units (S2, S4, S4+, SP10, SP30, SP31, SP35). I'm assuming here that since you've got the 120U of space, you've already got the cabinets, switches, PDUs, cooling, etc and would just need the mining hardware. That puts you in a great position (and to be honest makes me a bit envious
).
Your other consideration would be where you'd point all the gear. Pools like nicehash, westhash, f2pool, etc are all PPS with 3-4% fees. The upside of mining on this type of pool is you know what you're getting for returns. For example, nicehash charges 3%, so plug your hash rate into any online calculator (like
this one) and take 97% of what it tells you you'll make. Other pools, like kano.is, BTCGuild, p2pool, AntPool use PPLNS to pay you. They have a ramp up/down period but things even out. These pools are susceptible to luck swings. Find a few quick blocks, make a few extra BTC. Take a long time to find a block? Make a few less BTC. For example, AntPool's all time luck is 101%.
Another option is to rent out your gear. Services like
Mining Rig Rentals (that's a referral link, by the way... you don't have to use it),
Nice Hash, etc allow you to offer your mining equipment out to others. As you can tell from the fact that I put in a referral link, I use MRR. I also use NH/WH but don't have referrals setup with them. The idea with these services is that you point your miners to them, and setup a price you'd be willing to rent your gear. This way your gear hashes at a pool for you until somebody rents it, then you get paid for the rental.