Can't boot up - damaged hard drive?

Question

I've put all info here so hope it helps - 3 questions at bottom.
My system:
iMac G4 800MHz 80GB HD can't remember how much RAM but I think I'd put the max possible in it.
Mac os: 10.4.11
partitioned for 0SX and classic 9.9.2 (approx split 90%/10%)
The problem and what I've done so far:
The machine was running slow, then kept hanging. I checked and there was only 3GB free space so started to clear out some files (removing document files and Quicktime as I was going to reinstall it due to an error I'd received trying to open a movie file). The machine then either got stuck in a loop or hung trying to empty wastebasket. I moved some files from wastebasket to the desktop and moved a few at a time to wastebasket and emptied. Then it kept freezing, and I kept having to force a shutdown by pressing the power key for a few seconds.
During all of the above there was no (and thinking about it there hasn't been for a while now) startup chime.
I tried normal and safe boots and then reset the PRAM. Then it chimed on startup but wouldn't startup.
I've tried starting up from the original OSX10.2 disks supplied with the mac and a OSX10.4.3 disk. Only the latter will boot but only occasionally when holding down the C key on startup. If it does boot up this way it freezes after I get about 20% of the way through using disk utility or I try to install the system software.
Now either using disk utility or system software install, they do not recognise the disk.
When trying safe, single user and verbose boots, after about (est) 2-3 mins I get after about half a screen of text ending with:
diskOs10: I/O error.
Invalid key length
(4, 16111) -which I assume is the problem location on the disk
** volume check failed
syncing disks... Killing all processes
continuing
done
CPU halted -then the computer turns off.
As I haven't opened the computer up at all, I was hoping that it was a software problem that could ulimately be resolved by reformatting and reinstalling system software but with all the forced shutdowns and restarts (including having to restart with mouse key held down to remove/change installer disks) I fear the worst and think the hard drive is terminal or an outside chance that the superdrive is somehow at fault.
All of the above has happened in the last 24 hours.
I would like to try to at least copy off my email messages (100's) before giving up and replacing the internal hard drive.
Three questions:
1. Is there anything else I can do to try to reinstall system software?
2. I also have a MacBook (OSX 10.5.7) and a couple of LaCie 250GB firewire drives - is there any way I can hook these up or boot from the the MacBook or its DVD drive to see what's on the iMac G4 in it's current state?
3. Is replacing the hard drive straightforward or should it be done by a pro? (back in the day I often switched and replaced internal hard drives in old beige macs 6100's and as recently as tower G3's/G4's but I haven't looked inside a iMac other than to add RAM.)
Please help!!

Answer 1

Success! - this was my solution - it may help someone else avoid the three weeks of trial and error that I've been through.
I tried an older version (3.0.2) of DiskWarrior without success (web info suggests I'd need 3.0.3 or later) but I had managed to create disk images (.dmg) of the old partitions using DiskUtility onto an external firewire drive which I could mount successfully. At this stage I could to see all of my original files so rather than upgrade DiskWarrior on a 'maybe', I bit the bullet and reformatted the disk with the same size partitions and names as before in the hope that I would be able to clone from the dmg files.
I reinstalled OSX 10.3.4 and updated to OSX 10.4.11 to get back to where I'd started. I then tried using Carbon Copy Cloner to clone the disk images onto the new partitions but it kept crashing after a few hours. I also tried cloning to another IDE drive in another G3 (running 10.3.9) which also crashed. This left me thinking that the original problem had been that the hard drive was somehow in a knot and the 'knot' was still contained in the dmg file.
When I reinstalled OSX 10.3.4, I couldn't avoid using the setup assistant but as I didn't know a lot of the settings (I guess there are ways of getting this info from the dmg file but I didn't know how to and also, I wasn't sure if using OSX 10.4.11 settings in OSX 10.3.4 would cause more problems) I just entered basic details, restarted and updated to OSX 10.4.11. I then ran Software Update which updated a number of items.
I then used Migration Assistant to copy all of my settings and applications from the mounted .dmg file and it recovered most of my settings and applications. The applications work pefectly and didn't even ask for serial/validation details. I needed to spend an hour or two comparing two finder windows - one showing the mounted .dmg disk and another my new hard drive replacing newly installed Mail, Address Book and Safari files with the files from the .dmg file (Users, Preference and Application Support files etc), a restart or two later and it all appears to be working perfectly including all of my mailboxes, messages, contacts safari favourites etc .
Amazingly, it's a 80GB hard drive that I know was pretty full (showing only 3GB free) when I had the crash, the .dmg file is 67GB but now only 43GB disk space is used! As far as I know I haven't left anything out that I had on the disk before but as it all appears to be working well I really don't care!
I've already made new disk images!