Codename 47 is ridiculously hard. It shows its age. I'm on the fence about whether or not it is worth playing.
Contracts is excellent except that the story is fragmented (lots of flashbacks) so it is hard to follow. Contracts is very similar to Blood Money in gameplay. If you like Blood Money, Contracts is definitely worth playing.
I know there was 46s too. 48 was supposed to be the same as 47 without free will. Free will makes 47 the most lethal of his brothers.
I got all four games in one retail package. I don't know if you can get that in NL:
Edit: You know the Opera House mission early on in Blood Money? Contracts takes place immediately after it.