Between all of the TCers who have attended the rally we can prepare you fully for most of what you will encounter, with the obvious exception of the sheer weirdness of the event. There's little we can say to prepare you for your first "effing hell" sighting of the rally.
You can eat and drink on-site to your hearts content, but you may end up camped a trudge away from an Imbiss (bar) so a stove might be a good idea. And let's face it, Johnny Foreigner just isn't up to making a decent cup of Darjeeling at first light.
A thermarest mattress (or similar) plus a groundmat is essential to keep you away form the cold ground whilst you (attempt to) sleep. I take a camp bed and a thermarest. You can also buy straw on-site and that is good for insulation. You will need a pee bottle. I use an old Lenor bottle because the neck is wide (ooer missus) and it holds a fair amount.
As for the bike, pop every electrical connector apart, check it and reassemble using silicon grease or Vaseline. Moving parts like sidestand pivots and gearchange linkages should be lubed with something like chain lube. The road salt can start seizing these up even as you ride along. If you can fit legshields they will make a huge difference on the autobahn, the section past Regensburg can be ferociously cold and sticking your extremities into a 70mph airstream makes no sense.
When you get to the rally it might be wise to cover the bike (or the engine at least) with a nylon tarp or similar. Your poor bike may not like being left in the snow for a couple of days.
Don't just use a cordura goretex suit, once the outer gets wet it drags your temperature down. Buy a cheap (and baggy) nylon oversuit to go over the whole lot. I know it sounds obvious but a balaclava is essential (imho). I use a thin but warm Dainese balaclava that is very comfortable... I also have an FT one that isn't. Take a can of Bob Heath spray with you and clean your visor often.
When you are travelling on the autobahn, remember to take time to stop and eat, you need the energy at those temperatures. Get carbed up.
Andy M has all the gen on this stuff and I hope he will be along in a minute to direct you to his site, which contains lots of useful stuff on winter riding.
What day are you planning on getting there? I'd love to suggest "let's meet in Frankfurt and ride down together" but money and time are too short for a winter rally this year

GC