How much free space should there be on the start volume

Yes. You have enabled auditing somewhere along the line, being a wise person, but you have not trimmed the file, being a human, like I. Note the instant edit ;)

man -k audit

if you can get a shell will show you where you need to go

( look at them on the apple dev site )

i think here you want

audit -e 

to get rid of your old audit files.

From the man page:

The audit utility controls the state of the audit system. One of the following flags is required as an argument to audit:

 -e      Forces the audit system to immediately remove audit log files
         that meet the expiration criteria specified in the audit control
         file without doing a log rotation.

 -i      Initializes and starts auditing.  This option is currently for
         Mac OS X only and requires auditd(8) to be configured to run
         under launchd(8).

 -n      Forces the audit system to close the existing audit log file and
         rotate to a new log file in a location specified in the audit
         control file.  Also, audit log files that meet the expiration
         criteria specified in the audit control file will be removed.

 -s      Specifies that the audit system should [re]synchronize its con-
         figuration from the audit control file.  A new log file will be
         created.

 -t      Specifies that the audit system should terminate.  Log files are
         closed and renamed to indicate the time of the shutdown.

On my system:

ls /var/audit
20110414160828.20110415024057 20110420155533.20110421134946
20110415024148.20110416200834 20110421140348.20110421151928
20110416200951.20110417013744 20110421152018.20110422125418
20110417014358.crash_recovery 20110422125606.20110422205003
20110417024720.20110417120347 20110422205003.20110423102007
20110417135346.20110419142539 20110423102114.20110423204207
20110419144659.20110419150605 20110423204320.20110424094847
20110419150626.20110419201121 20110424094940.not_terminated
20110419201223.20110419225009 build
20110419225100.20110420153215 current
[root:8va:0:~ ]# du -h /var/audit
4.0K    /var/audit/build
6.0M    /var/audit

Yes. It's audit -e

[root:8va:0:~ ]# audit -e
Trigger sent.
[root:8va:0:~ ]# du -h /var/audit
4.0K    /var/audit/build
6.0M    /var/audit

You could make an automator action that starts each item serially, with staggered delays, or sleeps to ensure a delay between each start, and then simply place that single script as your startup item :)