Hopefully i can help a little.
1. How can I be positive about the price of a player? How can I know if I'm overpaying for him or not?
The only real way is to look on the market what other players of similar skills are being listed at / sold for. There is a market scout option, but it costs. In general, the more you use the market the more you'll get an idea of a players worth.
2. What should I pay attention for when I make penalty killing line ups? I use Aggression, but I don't know if this works for PK. Should I use something else?
Technique. Above all else that's the key skill i work with, followed up with passing and defense. There really is no defined "best" way, so it's managers preference.
3. Friendly games really help in something?
Yes. Friendly games build your players experience and chemistry, and also help you team earn OTR.
4. Playing games boost trainning? or a player can get the same amout of trainning even if not playing?
A player who plays a game will train less according to the guide, however the reduction in training isn't too bad, so i prefer to play over not play, especially if they need experience and chemistry.
5. Can the Forum be changed somehow? It took me a lot to find this thread.
We all wish that were the case

PPM really need to get a proper forum system installed, but all suggestions have fallen on deaf ears