I'm not sure, other than the tracks what would be lacking in the kit. PE for the 251s mostly consist of the external stowage boxes which are pointless if you will be modelling them closed and undamaged, PE is the way to go if you want to dent the boxes or leave them open. The PE kits might include the hinges and brackets for the door locking rods, but to use the PE would mean getting the plastic brackets saved and sanded off, and may not be worth the effort.
I would suggest you pull up a good photo of each of the PE kits and take a look at them with the model in front of you and determine which parts are useful as they would be an improvement over what the plastic overs. Then you can decide which, if any, PE kit offers you some value.
PE causes me problems, so I tend to avoid it when possible, my one tip for working with it is to bend up the PE part before you remove anything from the plastic that was molded on, if you mess up the PE part, you can always just go with the plastic.
The 251 doesn't have a ton of parts that PE would improve, other than the mention fender boxes, the visible part of the interior is the rear doors and the benches and the bins above/behind the benches and things like the rifle rack, most of that isn't really a PE type replacement, but more of a resin thing. The /9 might have a storage rack for the 75mm shells that is visible and would look better in PE, but other than that I'm not sure.
For the half tracks crew gear for stowage, inside and out, is always good. Stuff from your spares bin and perhaps a general german stowage kit could do the trick.
Gas and water cans would also be come tied or lashed to the outside of any of the half-tracks, along with cables, ropes, chains or tarps (do look at value gear
http://www.valuegeardetails.com/ for a pack or two of tarps, they end up being added to everything), there are some good PE kits for the MG34/MG42 that include better ammuntion belts (the crews were supposed to use the 50 round baskets or 200 round boxes to feed from, but would often link together hundreds of rounds and let it hang from the gun instead of in one of the containers) the PE ammo belts look far better than the plastic. Of course fresh rounds of 75mm would fill any rack and possibly extra rounds would be left on the floor, loose or in crates, and right after combat there would probably be spent shell casings on the floors, the ammo can be found aftermarket in both plastic and resin, and even with actual brass.
As to your specific questions Verlinden makes good stuff, you will need to trim and sand the resin (WEAR A MASK!) and can fill a vehicle. Almost all the gear and parts will work across the 251 line, the biggest change is from the a/b/c fender storage boxes to the d version. Other than that most of the stuff meant for one 251 kit will work on any of them.