The 251/22 would be an ideal kit to combine first.
I completely agree!
When I got the expansion, I was wondering why RM just didn't release this as its own kit (half-track included) because you only have the option to build the 251/22. Also, since I didn't have an extra 251D kit, I had to order another one.
The other bonus with being its own kit is that you can make a 251/22
or a standard 251D, so if a gamer/modeler couldn't find a regular 251 kit, they could still pick up the 251/22 kit and build it as a normal half-track. Almost like what you can do the Rubicon Models BM-13 Katyusha kit. I have two of these kits, but I plan to assemble one of them as a standard truck. My plan was to buy the regular truck kit (I needed another one), but the only kit available at the time was the Katyusha kit. Since I knew I could just assemble the kit as a standard truck (without canvas top), I bought it.
Note, the picture above is an older photo of my model, and was not finished yet (about 75% done).
I should take a new picture because it does look different now, and includes the crew figures.