Further to what I said above, I'll say that there are many ways in the early game to improve your squad at little or no cost - youth players, free transfers, pre-contracts, transfer list, looking for players who have a horrendously low market value for their overall ability rating, etc.
After you've done this for a season or two, and selling any (and I mean ANY) player you get a good offer for, you will be able to afford level 5 in every operating cost. Once you have this, you should really not let any of them drop, except if it's really worth it to bring in a truly outstanding player. I will say that the Doctor is overrated, though. I've had level 1 for 3 full seasons now, and only lost 1 player to a major injury that hurt me. A level 5 Athletic Coach single handedly prevented all injuries in season 3.
Outside your operating costs and wages, all money should be spent on absolutely top world class players. Good-to-Very-Good players can be got for very cheap if you look hard enough. And your youth team will flesh out your squad and turn up the occasional gem.
I'm at the start of my fourth season now, having won D2, Prem, and Prem+CL in my first three season, starting with a nothing club. I was trading at a deficit up to and including the end of my 7th transfer window (i.e. the money gained from my sales was greater than the money I spent on all fees added together up to and including the window AFTER I won the Premiership in my 2nd season AND the double in my 3rd). It is quite possible to do this, while improving your squad very quickly and getting your operating costs up to level 5.
In my 7th transfer window I have been able to spend more, as sponsorship money prize money has come in, but also smashed my record sale of 4.8 mil by selling Sturridge (age: 23, OR:75) for a massive 8.7 mil. I might have liked to keep him - he was developing nicely, but if an offer like that comes in, I think you've got to take it, - if brings you that much closer to a Gourchuff. When that deal comes along, I want to be able to afford it, and have room in my squad. I can't bid for him right now, as my team is still ranked well below 100.
