1) What is considered a good price for the S9?
it's $400-500 now in China, depending on the quantity and your resources, so I think that is a good buy? I don't know!
2) What is the most stable version? Is the 13.5 THz the 14THz S9J?
Not a huge difference, the ones with higher hashrate have slightly better chips overall, but the difference is too small to be considered.
3) What is the life expectancy that see those miners will have, since the coming months there are tons of S19 and other modern technology miners arriving in the market and the current difficulty skyrocketing?
Lifespan for all miners is really random and luck-based, it could be 1 month or 5 years, I usually take 6 months or less into account, I assume they will break and die by then, although I have some gears hashing just fine for 4 years, some died in 2 days, you can't really tell.
We create a whole topic for difficulty predictions, we all have different things to say, my prediction is a slow and steady increase in difficulty until year-end, no skyrocketing what so ever, check the topic by NFW to see my predictions and the reasoning behind it, keep in mind, I made more wrong predictions than I remember.
4) I have calculated the break even point at around 110 days in a price around $300 and the price of 0.06 cents. Does it make sense to purchase with that ROI?
If you can find an S9 for $300 ready to ship, that would be a deal given the current market price, at 6 cents you need this price mania to keep going, once things cool off you won't be profitable with an S9 at 6 cents, but until that happens, you should be good.
5) Is there a trustworthy overclocking firmware? I have read all the relevant topics from the forum but I didn't found anything that seems trustworthy. Does is makes sense to immerse cool them?
BraiinOS, Vanish, and the mods that thierry4wd make are the only ones I would trust.
And, forget about liquid cooling, it's economically useless, unless you get it for free it makes 0 sense unless you want a very quiet set up or plan to make some youtube videos or to impress the fellow ladies miners then anything except the default air cooling will come at a cost which will put you at a higher risk of losing more money, in plain English, ditch immersion cooling all together if you plan to actually make some profit.
From what I have read on other posts I think the same. S9 and j versions I think are ok, but I NotFuzzyWarm has a different experience.
NFW probably thought it's the K version which is indeed a terrible gear to own, you can see me "nag" about them
here, the S9j is a robust gear, nowadays it's even better to buy the J than the original S9 because the latter is a bit too old, the problem is, some resellers will swap the old S9 hash boards into an S9j miner or simply change the sticker, and you can't really tell the difference, this is why if you don't trust the seller you should go for the S9i which is as robust but has different dimensions and thus can't be faked.
. Thanks a lot, crystal clear and concise.