Unfortunately, I can't really help with players as there are too many variables such as what team you are playing as and what formation you are using. Basically I can just say what I look for in certain players.
Since this is a sports game, most positions you want players that have a high speed or "pace" rating. You can make up for a lot of mistakes if your players can more easily get back into position defensively. Also most players begin to drop in overall around the time they hit 30, or at least they will not usually get better at that age.
GK: The higher the rating overall, the better. Age is not as big a factor for GK's as physical ratings are not as important as certain mental ratings (GK positioning, GK Throwing, etc).
CB: I like tall CB's (6'0+) so they can better clear the ball from the box, with at least 70 pace.
RB/LB: I don't use these positions often because of the formation I use, but speed is very important, and size can be important based on your management style. They can also be used as RM/LM if they are offensively capable enough if your team is short in that position.
CAM/CM/CDM: Your central midfielders may vary again based on your management style. I would prefer to have a variety of different players for each position in order to have options for in game substitutions based on the game situation. Also CDM's should generally be taller than 6'0, but it is not always that important.
RM/LM: Your wingers should probably be the fastest players on your team, and should have a high crossing, and dribbling ratings.
RW/LW: Based on your formation may play as RM/LM, or as a ST. I usually keep them as the outside strikers in a formation with 3 attackers such as a 4-3-3 or a 3-4-3
ST/CF: I like to have variety when it comes to strikers or center forwards. I like some really tall strikers, and some speedier strikers to change my style of play based on the game situation.
It is probably a good idea to mix and match different player types with certain positions for more variety in game strategy.
I should also point out that a player's overall rating is not always the best attribute to go on to determine if a player is good. Sometimes a lower rated player will play better than a higher rated player for no particular reason.
This is probably very complicated, and I hope it is understandable.