The debate about when to buy players will always rumble on. Do you buy players at the beginning so that you have them from near the beginning of a season (thus training them as you like from the off), but risk the 'crazy market' at the time, or do you go for players when the market dies down around the middle and towards the end of a season, but will not be able to train a player for as long until he ages?
As for rating (assuming you mean quality), it all depends on how many tokens you have ready and you own preferences. The higher rated ones will always ignite a bidding war, while the lower rated ones won't have quite so much bidders but obviously won't be as big an improvement on your team.
For the SA, it depends on the SA in question, though if you train a player high enough anyway, the SA won't make that big a difference.
As you move up the levels, the quality of players you can purchase increase. The 21 quality player could probably still play a part, but any longer and you may want to consider selling him. As for higher than 25 quality, again it all depends on the server and market you have.
Hope this helps in some way.