Welcome to LQ.
architecture: See - What is Linux?
network media supported: Media? As in hardware? Check the HCL
clients supported: Humans? Not sure what you mean (networking wise - anythign that supports a protocl that is run by the system)
internetworking support: samba to comunicate with windows and apple talk for macs (is that what you wanted?)
services provided: Out of the box - httpd ssh ftp dhcp telnet (bad - evil) mta pop imap etc (anything in particular?)
protocols used: Just about anything that is standard and some that aren't
security features: RH 9 security Guide
scalability: In terms of upgrades (should take whatever you can throw at it memory wise etc) in terms of software upgrades - use multiple partitions so you don;t need ot format over data areas
hardware required: depends what you want to do with the system RH HCL
costs: Free if you download it. You'll need to pay if you want full support (Most people think LQ is better anyway
Most of your questions can be answered by reading the manual:
Just post any more questions you have.