Does anyone have any tips on where I could start with this?
If the server isn't to be used in a live environment soon, or if it currently doesn't run in production, why not just download a netinst CD and try it out? Just see how far you get and if you run into trouble, report back here with the relevant logfile output and error messages etc.
Are any of HP's supplied drivers necessary or are existing Debian drivers adequate?
At my work, we had problems with a HP fibre HBA, HP's drivers were better in that case. Most hardware should run OK, I guess.
I've already read the posts on the HPasm rpm2deb converter. Will this script install all of the necessary HP utilities?
Well, you might also want to install the HP System Management Homepage (HPsmh-rpm2deb-sarge.sh) to be able to use the webinterface and some tools.
Will I need to build a custom kernel to support their RAID controller, or anything else for that
matter?
This isn't too difficult, there are lots of HOWTO's on kernel compiling. The RAID controller is supported by the "cciss" drivermodule.
Does all of this work with the AMD64 version of Etch?
Errr, don't you mean IA-64?