as above...
for cabling you'll need cabling tools
for testing cabling, (either existing new or whilst installing) you'll need a cable tester.
cable management stuff, (tape, zip ties etc).
For network diagnostic, you'll need stuff like wireshark, traffic generators.
for fixing PC's boot disks, service pack CD's spyware cleaners virus scanners.
you might want to take images of a pc or server before you work on it. so something like Ghost or Acronis.
as well as CD's with service packs you might also want to get CD's with patches on them...
wireless network scanners, (to search for unauthorised/unsecure networks in a building).
PC auditing tools -to do hardware spec/upgrade and license management.
you can't assume anything.
you might have to fix a problem that needs a patch downloading but have no access to the internet so a 3g dongle might be needed.
and you'll also need a CD burner and a portable hard drive.
spare cables spare patch leads console connection leads for switches.
if you;re fixing old kit you may be just as well to carry old spares, (especially since you;re working from a shop) so that you can replace failed memory sticks, broken fans etc).
on the subject of memory sticks perhaps a memory testing program as well.
and probably a few thousand books depending on the setup that they use. I mean there are more mail servers out there than pretty much any one of us can name, so how do you intend to trouble shoot and fix problems in an unfamiliar environment, on an unfamiliar setup using software that you've neither seen nor heard of?