RLyon,
If you are working with your vendor then all you need to tell them is you would like to get your system upgraded to the latest version.
Depending on your hardware configuration, which they should know, they will provide you with your options.
For the most part it just requires the license to be upgraded to the latest version AND IF your Software Assurance is not up to date then that will have to be updated as well.
If you have a CS Controller then you will need to go to a HX controller.
If you have external Voice Processor then you will need to move to the Embedded Voice Processor and IF NEED BE a Processing Server - 1 [PS-1], but that would be a hard sell for me.
If you are not on the largest Compact Flash then they may want to update that as well, especially if you had an external Voice Processor as you will need the extra room.
There is nothing for you to look up or verify for them as the salesperson has access to everything and more through Mitel's Sales Assist Products which should tell them everything they need to know about your site and what you would need to be upgraded.
IF they insist that your provide them with a part number for any reason then get yourself a REAL MITEL DEALER that can take over your account with Mitel.
Thanks,
TE