How do I throw an awesome murder mystery party for myself and friends
in my home? (20-30 people, english manor themed if possible)
I want to throw a serious 'whodunit' party but I dont know how to
start. I looked online for kits but I found mostly cheesy dinner
theater groups that come over and do it. I don't think I want to hire
someone to come over but I'm open to the idea. I would rather find
some kind of "kit" that describes everyone's character and plot and
host it myself. Where can I purchase a kit like this? I dont have time
to write one myself but I wouldn't mind personally editing the
characters and plot.

http://www.murder.com.au/
is an Australian website (currently under design change or something)
has link to U.S. site.
http://www.murder-mystery-party-games.com/ 
downloadable kits advertised.

Just Goolge
Party Murder Mystery Kit

Have fun. My sister bought a kit, sent out invitations, I believe she
assigned the charters on the invitations and everybody had to come in
costume or they couldn't attend the party. She really enjoyed her
party as did her guests.

Great Idea!! Have FUN!