Well there are a number of ways to look at this and I'm not sure which answer to give.
Some things to consider. You will be looking at about 2000 hours build time for a Tomcat. If that is just weekend and evenings then the build will extend into years. There are a number of 'abandoned projects' for sale; this is both a warning and an advantage. You could use one of these as a starting point, altho' there maybe a lot of work that you wish to 're-do'.
Looking at the spec list you give so far it looks pretty good. You do have to ask yourself what sort of thing do you wish to do in the Tomcat when finished. The Isuzu engine is a good all rounder, but would not really sure any high speed events.
The LT230 transfer box would always be my choice. The VC can work well but are not know for their ruggedness. The R380 is a good choice too. I assume you have factored in a converstion kit to link the Isuzu to the R380.
With regard to the Disco 2 axle the wider track is only of benefit for high speed stability when off road. The internals are also expensive to upgrade and maintain. Additionally the Watts link used to locate the rear axle is great for stability but gdoesn't do much for axle travel. Again it returns to the 'what is your intended use' question. It does affect eh whole build

To be honest I would go with a RRC style chassis and axle combination. More availabillity of spares and upgrades.
Hard to say why you opted for the 4.1 diff ratio unless you are considering some fairly big tyres, in which case I would also look at getting a 1.6 ratio LT230 transfer box out of a D110 to maintain a reasonable off road low range. Of course if you go the ZF auto route rather than the R380 then you will be that this problem is somewhat negated.
My most recent experience in this area is with a D-Lander build. It's main donor was a TD5 D90; used engine, axles and suspension. The gearbox was an auto from a Disco2. This was with me for 6 months and has been returned to the owner for final finishing (cockpit interior and winch fitting)
Hopefully that gives you an idea of the scale of the project. All the thought and sketching and planning you do now costs nothing......but as soon as you start building, it can be harder to undo
