Type of bike for you (this might help):

http://bike.bikegremlin.com/test/test.html


Size:
Correct frame size


For budget and reliability, I'd go for a 3x8 speed setup (or even less 3x6), with good quality brakes (whether rim or disc). Shimano Acera / Claris or higher (or a SRAM equivalent). Like my commuters to have room for mudguards and racks.