So... you're not really giving up on mining with $0.15 per kWh? With all the maintenance costs and this price of electricity, I'd recommend you to get your solar energy system completed before increasing the number of your rigs (that solar setup should be your priority IMO). I gave up on mining due to having the power valued at $0.12 per kWh (it didn't really look like being worth it even at that price), and it's interesting to see someone else trying to build up a mini mining farm with an electricity cost more than mine; I sort of appreciate that!
About the wallets, it totally depends upon what YOU are going to plan with your mining revenue. Do you plan on trading them? Or just HODLing them?
There is good stories and there is bad stories. My personal experience has been quite good, started mining eth in 2016 with 13c a kw/h , paid power from my poket. And managed to make good few Zeross with only 6 mining rigs. Its not a luck, i belive harder you work luckier you get...ha ha ha