Unless you are referring to Defender XD's (I know the 90 isn't as well used nowadays but I wasn't aware they were selling the fleet just yet) they will all be NA D, the MOD stayed with this engine when the TD & 200TDi were introduced and it was only with the XD (Wolf) that the 300TDi was brought into service.
Chassis is the same as a civy one.
As long as you know what you are looking for it is still possible to get a good vehicle which has had a lot of money spent on it just prior to being cast, that said it is also very easy to end up with a real dog which will cost a lot of money to get properly sorted so as usual it is a case of buyer beware.
They are very basic and give you a (very) blank canvas with which to work on or if you like basic, a very simple vehicle to live with. The engine is no ball of fire but get a good one and it will go on for ever and return a decent 27-28MPG.
Unfortunately the days of going to the auctions and buying direct from the MOD are long gone but you still get a choice from Wytham's however I think the MOD now make the vehicles earn their living more than they did so they have see more service than those sold in the 80's, 90's & early 2K.