Yes it looks complicated. I've read up as much as I can on the SVA and I'm a bit confused, but it looks like if the tester deesm it to be "radically altered" then it has to be tested like it's a new car, the gov't doesn't want up driving old cars and this is another opertunity for them to get you off the road. So do some research, contact the local SVA station and have a chat before you start.
Unless you can keep the Disco chassi without cutting or welding it, ie the whole mod has to bolt on to what's there already, then I think you are looking at an SVA.