Single breasted – double breasted is for old tubby men to get around the girth.
Eh? Eh? EH?
Double breasted makes you look bigger, and should only be worn if you're a bit of a racing snake. And you're a lawyer in a 1980s/90s film.
Single breasted jacket, 2 or 3 button depends on fit – In my view, I look better in a 2 button. 3 Button to me is for skinny blokes with two button collars. Who arrive by moped.
As others have said, Navy or Charcoal Grey are decent colours to go for. Some people will question the wisdom of black shoes with a navy suit, but I feel that you need black shoes for an interview, so grey would be a safe route to go down. Interview clothes need to be safe and dark coloured for most industries.
I agree with everyone else about a black tie with a black suit for interviews. Not good. Get a decent but fairly sombre tie – stripes or paisley if you like, but in a sane colour.
M&S do some of the best suits on the high street according to one of the Saville row boys who has his own blog. Sales are fairly regular, so if you can, try and time your purchase well.