Having just visited the Warbirdsim site, I've come back here a little confused (yeh, yeh, I know, nothin new eh?)

Why are there soooooo many 'Ds' ??Can I just purchase the latest and greatest and, if so, what am I missing without the earlier releases?
Dougal, you could be in for nights of reading each product page, to discover why there are four different D-model products. ; )
At the risk of sounding like a broken record -
The basic thing you have to understand, is that there wasn't just one "P-51D". Changes continued to be made, right on the production floor, throughout P-51D production. Every time a significant number of changes were to be incorporated, the production model changed. That is why it really does mean something, when you say P-51D-5-NA, P-51D-10-NA, P-51D-15-NA, etc... There are long lists, as originally written by North American Aviation, as I have in copied form, that document all of the changes between each of these production blocks. There were even numerous changes right during the middle of production block, making for 'early' and 'late' examples of one specific production model of the 'D'. (Just for
one example, heading into P-51D-20-NA production, the P-51D had the N-9 reflector gun sight, no support for rocket mounts on the wings, and no tail warning radar, which meant that the cockpit electrical panels were of a type that reflected this. Part-way through P-51D-20-NA production, the K-14A gun sight was fitted, the wing was modified for the support of rocket launcher stubs on the wings, and a tail warning radar set was installed, requiring rather significant changes and additions to the cockpit, to provide rocket-firing capability, and the warning equipment for the tail radar.) Many don't know or realize, but the P-51D-5-NA to the P-51D-30-NA, is very much like the Spitfire Mk.1 to the Spitfire Mk.V - in both cases, there are several hundred changes, both minor and significant, between them. I can understand why it can be so hard to see this, as for years, every P-51D production that has come about (no matter it be the plastic-model industry, or flight simulator), ends up being very 'generalized' version of what someone might want to call a "P-51D", because the company did not want to, or maybe did not know better, to try and reproduce specific production block variants of the P-51D. For instance, producing a P-51D-5-NA, how many times has a company just gone about it, by taking a generalized P-51D-25-NA or D-30-NA (which are models of the P-51D, so late, that very, very few ever saw combat in WWII - but are the most predominant examples surviving today), and just knocked the dorsal fin fillet off. I'm sure that is fine to some I suppose, but it doesn't come close to making it a P-51D-5-NA (if you care about the details). Of course, today, with various restored Mustangs flying throughout the world, there are even more changes and unique differences from one example to the next, besides the differences that are traced to the aircraft's specific production model.
Starting with the Restored P-51D products, I wanted to reproduce examples, exactly as they are today, not just by creating one P-51D model, and changing the paint on them to match, but actually modeling and texturing each, inside and out, to match each example portrayed. This means that really, in just the "Restored Part 1" product alone, there are seven uniquely different aircraft (rather than one, with repaints). Each cockpit is different, matching the look and layout of the cockpit on the real-world example (as close as what could be accomplished in a reasonable amount of time).
In "Restored Part 2" of the P-51D's, the same was done as with on "Restored Part 1", but 8 more restored examples were selected, to be reproduced in the same manner.
With "Little Friends", this was the first product focused on WWII examples of the P-51D/K, featuring mid-model and very late model examples, with original WWII-era configurations and weathering to match the original examples depicted.
"Little Friends II", created as a continuation to "Little Friends", focused on just reproducing the P-51D-5-NA, to bring this, the earliest model of the production series of the P-51D, to FSX form (and by far the truest reproduction of the type for any simulator - which can be also said about all of the various P-51D-models produced by Warbirdsim). The amount of work required to do this, necessitated it being a dedicated product by itself.
As I have mentioned before, as time goes along, through added learning and experience each product is bound to get better, leaving the most recent better than the past. "Little Friends II" is the latest production released, but "Little Friends" is at the same level as it, especially if you download and install the "Little Friends Redux" pack that I produced a few weeks back, and for which you can find here at SOH. The "Restored" packs are showing their age a bit, in comparison, but I'll be working on fixing that over the next month or so (of course the results of which will be free to customers of the Restored products).