There's some pre-soldered ones too. Just search on ebay for "arduino nano" sort by price and keep scrolling until you find one.
If you get one with USB built in you don't need a USB/serial adapter to flash it.
Totally depends on what sensors but loads are 99p (from China).
I recommend buying some small cheap breadboards and jumper cables.
CPC are good for cheap leds, resistors, transistors etc.
EDITED: 14 Jul 2014 08:24 by ANT_THOMAS