According to the strategy sites, the ideal would be to play at levels where you have at least x100 in buy-in, that is, if you play at levels 0.05-0.10 you should have at least $1000
100 buy-ins for a cash game will be quite enough, although I personally prefer a bankroll of 200-300 buy-ins, with which I feel pretty confident even with long downstreaks
This is a very important issue and a well-thought-out approach is needed here. I recommend that you set a spending limit per day, such as 2-3% of your bankroll per day, and always stick to this limit.
I disagree with this statement. There's something called risk of ruin. The problem with playing 10-15 buy-ins is that even though you may be a profitable player, poker is a game with very high variance. That means that it is incredibly probable that in the long term, you will go through substantial downswings, even if you are playing profitably. If you are at ~100 buy-ins, the risk that you will lose so much that you will be unable to continue playing is relatively low. However, if you are playing at a stake where you have ~10-15 buy-ins, you are almost guaranteed to get to 0 eventually.