Can you suggest a computer (PC or Mac) that’s dependable, priced at $0-$300, and will not be obsolete soon?

Well I can see that someone has been giving a lot of thumbs down to answers here, but here is the truth. You are simply not going to get what you want. I am terribly sorry if this is not what you want to here, but the truth is the truth. The cheapest computer I have seen is one that Dell is offering for $359, and that does not even include a monitor. You throw in the fact that you want one that will be useful for many years, and your request is simply impossible. Now I don't know who is giving all these thumbs down to the above answers, but this is not a case of people being unhelpful, they are legitimately trying to explain the situation, what you are requesting simply does not exist.