Its pretty easy to upgrade from 5.2>5.3, you just need the media, and select the upgrade option when booting from the CD/DVD media.
You will have to plonk a VDU on the machine, and a Keyboard, as you have mentioned you have no HMC (This is more useful than stated, as you have a console connection to the machine =)
If you have a HMC elsewhere on the network, you can use it to manage the machine in question, providing it has a network connection.
Its a pretty simple migration, but have you considered the compatability of your H/W?
I'm pretty sure 5.3 drops support for a version of the PPC, I think it drops PPC3, so if you have an older box, that could be an issue.
Have fun!
Ryn.