I can add from my experience over 11 seasons, and a further 10 on my second team, that having older players in your team has no effect whatsoever.

I have had players who have been 28+ in constant rotation with 18 year olds and they have performed exactly the same.

The only thing that changes with an older player, is a reduction in skill gain with training.

I know I probably come across like I slam this game all the time, but the game engine isn't intelligent enough to take player 'experience' into consideration, but I can understand why some people may like to 'fantasise' about things having an effect as the game does seem very basic at times and can lack the realism of real football.

In regards to the OP's questions, as the first two have been answered, I find that the best time to find players is at non-peak times.

I can snap up a good amount of 5* 18 year olds only a few days into the season by bidding at 2am, and other late hours, as most of the people on my server seem to be European, like me.

It's true that they're harder to buy as they're more desirable, but it's worth a shot at getting them, it helps keep your team stable as the players will last a good couple of seasons, rather than having to offload an aging team every season as they've dropped stars and won't regain them fast enough.

Don't settle for second best without giving it a try.

But that's just my two pennies worth, there's some great guides in the useful links thread on the forum, in the guides section.