Pressing the C key does work have any effect on an SE/30. The key combination that one in certain cases could use to bypass the internal hard disk would be Command(Apple) + Option(Alt) + Shift + Delete(Backspace).
However, with a proper startup floppy (such as an original System 7.1 Install 1) in the floppy drive, the computer should automatically start from that disk. The floppy drive has priority over the internal hard disk at startup.
I agree with JustSomeGuy that making the machine ready for IP networking is going to more complicated with an earlier system than with 7.5.x, but it can be done. Remember, MacTCP is not part of System 7.1 (it has to be installed separately).
System 7.5.x installs the PC Exchange control panel, which makes floppy transfers from a PC easy (if necessary). System 7.1 has an Apple File Exchange program on the Tidbits system floppy (this program has to be installed manually) for the same purpose.
Yes, System 7.1 is faster than 7.5.x on any 68030 machine.
If there is a problem with your set of System 7.1 floppies (which are not provided for download by Apple), you could download System 7.0.1 instead:
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/System/Older_System/System_7.0.x/
The British version of System 7.0.1 is available in the form of separate floppy-sized downloads:
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -British/Macintosh/System/System_7.0.1/
Use Disk Copy 4.2 (the Make A Copy button) or 6.3.3 (the Make a Floppy command under the Utilities menu) in order to create correctly sector-copied floppies from the disk images.
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/Utilities/Disk_Copy/
Jan