I can help you for sure if you have a specific position you want to buy a player for and a budget. That would be fun. But, as they say, give a man a fish and he eats for a day, so I'll give you two bits of general advice...
1.) In edit mode you can highlight each statistic for any player and see a definition of what each stat really does. Making good purchases without really knowing what you're buying is very difficult! I think a lot of people don't understand the stats, or even the player/skill cards. If possible, also try to get your hands on a copy of one of the old piggyback interactive guides for PES. Most of the stat definitions haven't changed, and even having one for, say, PES 6, will provide a lot of useful info. For example, most people know that there is a big difference in effect between a stat rated 75 rather than 74 when compared to 73 rather than 72, in other words the number 75 is a break point where there is a big difference in the stat's effectiveness, but did you know that the difference between 95 and 96 is also a big leap? According to the guide, all stat points above 95 are very effective, so being 98 as opposed to 97 is a real difference. Of course, if I have a player who is 95 at a key stat like shot accuracy, I'm happy enough, but I'm just using this as an example of the kinds of things you can learn from getting a copy of one of the old guides (I don't believe they make them anymore).
2.) I base my evaluations of what is important for a given position based on playing the game itself, and also on my experience with real life football. However, if you want a base understanding, go to edit mode again and try messing with existing players' stats to see what does and what doesn't have an effect on overall rating for a given position. For example, try taking a central defender and giving him 98 shot accuracy and you will see his overall stat doesn't change. Give him 95 defence or balance (strength) and then see what happens. This is the best way to understand how the game determines the overall ratings, but what I'm trying to say is that the best way to make purchases is to actually try to exploit the simplicity of the rating system. So, again, the overall rating is important, but it can be misleading.
One last thing, just as an example I'll explain how I search for new "gems." Lets say it's an attacking midfielder I'm looking for and I only want to spend, say 2-3 million (or whatever)... I will do a search and select AMF as the position, then go to abilities and try to define what are the important parameters I'm looking for in that role. So, let's say that I have been playing for a while and realized that my midfielders aren't connecting on enough passes to the striker and I want a player who has good passing. In that case, the abilities I want to be good are pass accuracy, pass power (also defines if the pass is effective), and teamwork (helps passing by making good connections between players). Furthermore, there is the "pinpoint pass" special skill and also "all out attack" which makes the players off the ball make forward runs when the player with the card gets the ball. I might like to have some of those skill cards but I'm looking for a pretty inexpensive player, so let's consider those a bonus.
I'm using PESTOME, an online database, not the actual game so this might have some errors, but I would start by entering a value of greater than 80 for short pass accuracy, and also 75 for short pass speed, dribbling acc, responsiveness, stamina, ball control, and 70 for balance and shot accuracy (and a minimum of 4 for form) then see what it turns up. Of course, the things I look for are not the same as what everyone else would look for. A lot of people look for speed and acceleration first, and those things are important for an AMF too, but I'm just giving an example here based on a player who is just starting out and wants a passing-minded AMF. Maybe next he would search for a stronger or faster AMF, but that's up to him.
Anyway, once I have this list, I will sort it by overall rating in reverse order so that the lowest overall players are at the top of the list. This, in and of itself is a good start. Right away I have the cheapest players who I could buy for that role with the basic stats that I want. Ludovic, one of my favourites, who is no longer in the game for some reason, is that the top in fact!
The next thing I would do though is go through the list and look at the players one by one to see who I want to actually buy. Sometimes it helps to save five or more players to your favourites list, then work through the short list to make your final decision.
Immediately I see that there is no player with 85 short pass accuracy for really cheap and who has these other stats I want, so I will look next for if any of them have useful special cards... Unfortunately though there isn't a player who meets those conditions who is less than 75 overall, so I look at other conditions. Note however than if you have a bit more cash you can buy Di Natale and get some great stats for a price way out of line with other players (not based on market price, based on overall base stats price).
If you have a bit more Lucho or Sorlin are good options for this sort of player, but if it's a cheap player you're after then look at the likes of Devic, Zortequa (85 teamwork and good strength for an AMF), Abriel, Danilo, Toscano, Chantome, etc. Cabelaard might also be a good buy, but his body balance is low so I prefer others instead (don't want to get bumped off the ball too easily).
Hope this helps, but if you want I can look at something more specific. I'm not playing the game right now (lent it to a friend) so this lets me get my fix
Peace
