If I was buying, I'd go for the old 200s/300s.
Agree, the 100s are very much super GI clubs and are cast too.
Yes. I had a set of "entry-level" cast Mizunos and they were nowhere near as good as my 900s.
Having said all that, I still reckon you've either got to be brilliant or really quite rubbish to notice much on the course. I have days (not recently!) when I miss nearly all the greens and salvage a few pars to play to h'cap - I can't see the clubs make any difference. 
