I think choosing staff depends on how serious you are. Most new players just tank the season pay the minimum salary and not invest at all in choosing the proper staff. If you do decide to form a competitive team and discard most if not all of you scrubs read the part about training in the faq section of the help forums. In my first season, the players i picked were really cheap (they were really good players with atrocious stamina) so i didn't need to invest in a trainer (just a basic level fitness was fine),
with the doctor" basic" for me was fine because the competition in Div V was not that great. I read somewhere that injuries happen more often in the higher divisions. Though to some extent they are still random. I think my advice is determine first what kind of players are you going to buy before deciding on which kind of trainer to use.