It is up to you where you buy from but you can look in the Pelican Parts Catalog and see the Glow Plugs and part numbers you need. Use only Bosch or Beru Glow Plugs.
If you get the correct plugs the voltage should not matter. Unless of course you think you are getting too much voltage and that is killing your plugs.

I have not read stuff on the newer models as you have to know if they Glow Plugs get stuck in them as sometimes happens on the other diesel models that have aluminum cylinder heads.

So there could be a tough choice. Change all of them and risk snapping of one or more what were working glow plugs in the Cylinder head or limit the risk of so many snapping off and only replace the bag Glow Plugs.

Once a Glow Plug is out you should ream out the Carbon with a reamer or some other alternative method.

When the glow plugs are installed use some never-seeze type compound n the threads.

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84 300D, 82 Volvo 244Gl Diesel