1. The power usage is based off of the power usage in lab conditions running their 93%-efficient APW3 PSU. Now, assuming you don't use those, you're going to get a small bit more power usage due to the fact that most psus are not as efficient, though some do come close and beat the AWP3.

2. Difficulty is not easily calculated, and it will occasionally drop like it is now, or rise just a smidge like it did for a while. Assuming you buy right now and it gets delivered right this moment(which is impossible), you'll be off to a good start with the lower difficulty that will come the next difficulty correction period.

3. S7s are generally cooler and more reliable than S9s imo, but really it's up to you which one to buy. S7s have held their own for quite some time and have had lots of users, while the S9 is slightly less documented than the S7. it's your pick.

4. Generally, I use old miners until they make less than 3.5$ a day. With your cheap electricity, you may want to sell a bit earlier, as your electrical costs are comparable to some chinese miners and people with nearly free electricity. This means you may want to sell early, so that buyers are more inclined to buy the miner; they'll make at least a bit of profit.