Which G is it supposed to be?

Hello together! I am new here in the forum and look around to buy a G. I was already at the friendly dealer, but no one can give me any information if the AMG rims with the extension of the G55 are also available for the diesel. I have chosen the G 320 CDI long, in Designo Lack and designo leather. But I still doubt whether the 320 is the right choice. G55 or G 500 are leaving, because I don’t know any gas retrofitter and no experiences about it. I once had a ML 400 CDI of which I was very enthusiastic. However, I always hear and read about problems with this engine. I had at least none, but have only 50,000 km of experience. Because for me a top used G 400 CDI could be considered, but I can no longer influence the equipment and would probably have to look for longer… But the V8 diesel would like. The 400er is still in the “older” equipment, with the “old NAVI”. Is there something seriously negative? I h with the ML Bj. 2004 only with NAVI and telephone problems, but then enough of that. In addition, it was a CD-Navi. What do the experts say here?? Greeting mw P.S. Of course I would like the G55 best. Wouldn’t be the problem financially. But I just don’t have to fill up every 400 km…if that’s enough!