I think the important part, is to make sure that whatever plinth you choose, that it has the corner bracing.
Any new plinth will do. If you have an old plinth without the large corner braces it's at least forty years old so if it's still straight and tight today it's probably going to stay that way. Sonically, I don't think it makes any difference.
You can still get fluted plinths. There are a few aftermarket options and Linn still do them occasionally, I have one. It might be an option on the plinths I'm sorting out but I'm wary of it. Lots of cuts and if you get a chip the plinth is scrap.