The V55/5 form is for the registration of your vehicle. The info you put on here will appear on your V5 (but not all of the info will be on it) You will need to fill this out with as much info as possible. The guide to filling it out will state what to leave blank (which is most of it) If you haven't got a chassis number then I would think you will need to put down the hull number in it's place. Question 46 will ask you if it's Type Approval exempt. You answer Yes to this as it's an Armoured vehicle and also because of it's age. If it's over 3,500Kgs you will need to answer question 49, which asks if it is exempt from the Goods Vehicles (plating and testing) Regulations 1988. You will need to tick YES for this to be recorded as an MOT exempt vehicle.
The V112/G is a decleration of MOT exempt status, you fill this out and hand it over at the time of registration to declare that it's MOT exempt. The DVLA will keep this form.
The letter you have confirming the date in service will be the evidence DVLA use to issue you with an age related number plate.
If your lucky, everything will go smoothly and they will issue you with a registration number there and then and a Number Plate authorization form for you to get your reg plates made up. You hand over the money they want and that's it. You will walk out with a Number Plate Authorization form and a Tax Disc (which in your case will be free:-D)
Then in 12 months time when it's due another tax disc, take your renewal form along with an insurance certificate, V5 (just in case) and a completed form V112/G down to the Post Office where they will issue you with another 12 months free tax. Once you walk out of DVLA that's it.
It does all depend on who you get at the local DVLA as to how smooth the whole process is.