beginner question : differences between some clays

Hi, i've used water-based clay for a time but i'm totally new to
sculpting polymer clay (is it the same as 'oil-based clay' ?), and i'd
like to order some to give it a try.
But i'm quite lost between the different qualities available. I'm not
interested (a priori but again, i'm not sure) in sculpey, for i'd like
to do some casting and also it seems to be more expensive than other
polymer clays.
As for "oil-based clays" I've been recommended "roma plastilina" or
"chavant" (which if i understood is the same with less risk of alergy),
but i can't seem to find a resseller (i'm in belgium) and providers i
found were in the US and didn't deliver to Europe.

I've read about Fimo, Premo, kato, but didn't manage to figure what the
differences were with chavant and roma plastilina.

Could somebody point me to some comprehensive ressource (web site,
reference post, or book) for i'm at a loss.

NB : if there are europeans reading, where do you buy yours ? It seems
to me quite an expensive solution to have it sent to Belgium from the US
anyway.

Thanks in advance !