Lubuntu minimal install?
Instead of using CD's you could try network-boot the machine on the minimal ISO (if Lubuntu comes with netboot stuff)
I would then install Lubuntu to the desktop, followed by rebooting to the desktop hard drive and running
sudo apt-get install lubuntu-core
You may need to modify /etc/apt/sources.list to a non local server and then run
sudo apt-get update
You may also want to use
sudo apt-get install lubuntu-core --no-install-recommends
However I have not tried this.
Yep, you can use the mini.iso instead. It's like 32mb total and it does an install similar to tasksel like Debian or Ubuntu server where you can choose what to install and it will install it. This also allows the option to not install all the bloat you normally get with a desktop install.
BTW, I do not believe you should install any of the "live" desktops listed. I believe those are not for installation purposes.
https://help.ubuntu.com/community/Installation/MinimalCD
I believe 14.04 installs a minimal command line environment first, then reboot and type
tasksel
14.10 and newer have a better interface that lets you choose the desktop or apps to install right away. Just choose command line install or graphical install (I think they are actually the same).
NOTE: because of a bug in tasksel, DO NOT use tasksel to uninstall packages. ONLY use tasksel to install.
UPDATE
I think I found what you are looking for, it's called "alternate install" for Lubuntu and it's for pcs that can't handle the full Lubuntu version and or need to install using a CD rather than a DVD or a USB. The iso is 617mB, however, when you burn the iso to disk it should fit within 600 because the 617 includes reserved space, title of the file, etc ...
http://cdimages.ubuntu.com/lubuntu/releases/trusty/release/lubuntu-14.04-alternate-i386.iso
Also, other processor versions (for older processors) are available at the Lubuntu/Alternate_ISO wiki.
Finally, if you don't have more than 700mB of RAM, you should select the installer from the first boot menu and do not select the installer from the "try Lubuntu" interface as the "try Lubuntu" option uses 200mB RAM to run the desktop.