First things first. A block of bitcoins is only 12.5 bitcoins, it has not been 25 for a long time.

Solo mining has a worse chance than playing the lotto to find a block with a single machine. You will just be throwing money away.

Comparing GPUs to ASICs is apples to oranges. You flat out dont mine bitcoin with GPUs, thats what all the other alt coins are for.

Also with just a single machine you can mine on dial up. You need almost no bandwith to run a miner.

Dont use nicehash to calculate profits. IMO dont use nicehash to mine bitcoin either. Nicehash is for lazy people that dont care about maximizing the money they make.

A brand new S9 from bitmain is around $1300, if you are paying $4000 you better ask for some lube while you get bent over. Also from a third party you get zero warranty. Dont do it.

The s9 will be around for at least another year, probably 2 before something usurps it as the most efficient miner available. If you dont pay some outrageous cost for them, you will make your money back. Even at a high residential power rate an s9 will make over $300 a month in profit and pay itself off in under 4 months.