It sounds like you're pretty sure your server will eventually start and you just want to make sure that it's ready before the client starts so that your bash script doesn't fail.

That's not so easy. How about this, make sure that your client returns a proper status to the shell based on success or failure.

#!/bin/bash

SRV=/full/path/to/server
SRV_ARG="-nodb -co /full/path/to/server.conf"

CLT=/full/path/to/client
CLT_ARG="-user=admin -p=23000"

$SRV $SRV_ARG  $@ &
sleep 3

if ! $CLT $CLT_ARG ; then
        sleep 3
        if ! $CLT $CLT_ARG ; then
                sleep 6
                if ! $CLT $CLT_ARG ; then
                        echo Server must be broken!
                        exit 1
                fi
        fi
fi
[download]

You'll probably need to work with more than a single status from the client. Maybe you'll have RC_OK, RC_NOSVR, RC_BADARG, ... and your script will have to check for RC_NOSRV and try again.