I can see and respect the idea of buying cheep if it's just going be a 'run around' sort of bike. But as others have said you still want to enjoy the ride and feel like you are happy to be seen riding it just because its riding to work it's still riding a bike. You are going to have days when you don't feel like using it last thing you need is a bike that you don't like at the best of times. One thing to also think about is riding a heavy bike is one thing but moving it around once you get off it is another depending on your home and work storage access as one example. Also buying new and being new to cycling you have some security that it's not going to brake after the 1st few miles.