Congrats on the new purchase! I build camper vans professionally and have removed hundreds of factory interiors from eco online vans.
Personally I would remove the original plastic wall kit.
The factory insulation does not provide you with very much protection from the elements. When Ford was designing your van, they were not planning on you sitting in the car with the A/C or Heater turned off, I.E. the factory insulation is typically just enough to maintain comfortable cabin temperature while the engine is running, but once you turn the engine off, the vehicle will get hot/cold very quickly!
You could insulate the van 10x better with some spray foam or fiberglass insulation. I am recommending those two insulations specifically because they are highly mold resistant, which is always a concern if you ever plan to camp in colder environments. (I live at 10,000 feet and am constantly struggling to keep the inside of the van dry, I have a dog in the car too which creates extra humidity).
Another great benefit to removing the factory wall kit is reduced road noise, the plastic wall kit will expand slightly if exposed to sunlight over the years, which causes the plastic to creak and make all kinds of annoying sounds whenever you hit a bump or you’re on a rough road.
Alternatively, if you just want to cover up the look of the plastic walls, I typically recommend using automotive interior carpet, you can apply it directly to the plastic walls using a web spray glue (3M sells it at Home Depot). Silver Automotive Trunk Lining | OnlineFabricStore