You could easily run all those services on one box running a linux distro. About the security thing, all boxes are insecure in some way. I would say read up about security issues with your distrobution and services that you want to run and monitor your system. Nothing is secured to perfection, but it will be a lot more secure to avoid an attack of some kind. You dont need seperate computers to run each service. (and anyway wouldn't that be very expensive??). For some how-to's you can go to...
http://www.linuxdoc.org/
I hope i helped you a little bit.
-Mike