power reserved for asics: 15.000w in one place
used gears? i don´t understand that sorry
we call mining asic "gear", I hope that is clear enough by now.
The problem with your request is that you have a good amount of money, free power but 15kw is really too small, in order to fully utilize the free power aspect, you need to be using cheap gears that are less effienct, such as antminer S9s, but then with that small amount of power, you will only be able to have just about 10 of them, which is a total of 140th.
Including racks, wires, and all, that will be in the 7k range, which is nowhere near 60k, but then 140th is really not worth the efforts IMO, yes 10 S9s will get you 1.2k monthly based on today's prices, say you will spare $200 to buy replacement hash boards/PSUs, so maybe you net $1000 a month, but I think the current figures will be the best you will ever see, in other words, this 1k isn't sustained by any means.
the other approach would be getting efficient gears to get the maximum hashrate squeezed in those 15kw, but then you lose that "edge" of cheap power since those very effienct gears don't require much energy to run.
My pick would be somewhere in between, 40-50w/th gears will do, maybe a second-hand M20s, uses 48w/th , does 70th and costs about 5k, you can probably squeeze in 4 of these, the total cost should be in the 22-23k range, hashrate is about 300th, that's about 3k per month, you should assume that the average income for the next 2 years (assuming the asic won't die on you) will be half of that, if the price does not collapse, the difficulty will double, so any expectation above 50% of the current numbers is somehow wishful thinking, so 1.5k a month.
But then you said:
Mining isn't really a very long term business, it's not something you can retire on, I mean in a few years a few halvings will cut the rewards into pieces and there is no guarantee that either the transactions or the price will compensate that, mining is really just investing in BTC using a different model than exchanging cash to bitcoin.
So my humble opinion is that nobody should count on mining for their retirement plan, health care, student loan repayment, or anything that has no room for error, mining is a very risky business, it should be treated as an opportunity to maximize your gains in the short to mid-term and nothing more than that.
You could potentially turn 60k into 120k in 1-2 years if everything plays out well, but will you be making 60k a year mining bitcoin for the rest of your life? most certainly not.