ET refers to the offset, I think it means something in German. This site gives a good explanation:
http://www.tyresave.co.uk/choose.html#FITMENTNormal RS and RM split rims are 6.5" wide. Anymore than that were ordered wider or modified afterwards with wider rims.
I think standard VW offset is 32 or 37
195/50 would be the standard and cheapest tyre option. However 195/45 visually look a lot better without making a difference to the rolling radius.
A set of RM's are probably ?350 - 600
A set of RS ?500 - 1000 the latter for superwide polished ones
You can't get new ones of either model as they have not been in production for years!
Hope that helps

jv