I have free electricity and have been thru this debate many many times,
If you buy expensive newer miners then it will take longer to reach ROI but you can host more of them since the available power is capped. The ROI on the S7 is like 8 months or so.
If you buy older and cheaper miners you have a shorter time of getting ROI but you won't make alot of money. The ROI on the S3 is like 2-3 months or so.
The more into the future you go, the higher the uncertainly. Price of BTC and Difficulty is easier to predict in 3 months then in 8 months.
So you need to find a balance in between.
Personally I wouldn't buy the S5 or SP20, they are overpriced at the moment. Get the S2, S3, Dragon Miners, KNC Jupiters, etc.
You will make the same amount of money in 6 months with an S3 as you will with an S5 but the S3 is 1/4 cheaper.
I only made money with my miners because I profited off somebody else's loss. I only bought miners which were no longer profittable due to electricity cost or I bought them because the price of BTC was crashing and Difficulty was skyrocketing and the sellers panicked and sold to me.
Only way to make money in this business. Its a zero sum game pretty much. You're gains will be somebody else's loss.
That's why I wouldn't buy an S7, because our loss will be their gain.
I don't know on dragon miner's. They are getting close to end of life. They run hot and are not efficient. Going with S3 would be much better idea with free electricity. Also I think S3 will have more value if you ever to to sale it say 6 months from now and upgrade.
S5, SP20, Avalon 4.1 all have held value pretty well with new miners slowing down. So if free electricity it will be harder to justify but they really are great machines.