About Operating Systems security

Hello everybody,

First of all, I know the subject I'm going to talk about has largely been discussed everywhere but, up today, the main problem I have is that I can't really find the right Documentation I'm looking for and as much as I read reports, the task become harder to do. So, now, I ask for some helps to the list...
Ok, here I start. Considering the following fonctionnalities installed and the same machine and nothing more :
(a) a firewall
(b) a web server
(c) a database

I have already prouved that the security level will be the highest if I use OpenSources for (a,b,c), and for reaching that point, not only the security process and procedures has already been written (Process and procedures for regularly auditing the fonctionnalities installed and also for dealing with a recovery plan, for exemple) but also the code and reviews that could be done or have been done.
But as (a,b,c) is supported by an OS, the hardest problem I have is how to introduce a new one in a companie - Probably I should have started to think about that before...- How to prove that the OS choosen for only supporting the fonctionalities ennonced before, will be the most secured OS between all. How to prove that it in front of directors, managers and Engineers.

Any pointers, news or documents are welcome and I'll keep everybody informed on the result :)

-Yannick