I recently bought a bike, second hand, for cycling to work and around and about. I bought an Orange Gazelle, sit up and beg style bike, for £175 as I constantly had neck ache with the forward lean required on my other bike which is a 1980's "Milk race" with drop handle bars ( bought that at car boot for a tenner and had it serviced , total cost £30, and ten years good use from it so proved to be a bargain).
Depending what you're looking for I'd recommend one of the Gazelle bikes, not the cheapest but great bikes and comfy to ride. If you can get one second hand at a decent price they'll give you years of service.
As for the getting fit, I'd not cycled for a year or so but soon found I'd no need to keep looking for the lowest gear at the slightest incline. Another plus is you tend to see so much more when cycling, I still get a thrill at cycling past the Minster here in York and taking in the view! Invariably it's also quicker to get around this place by bike and I love the smug feeling as I cycle past the queuing cars on Bootham and other notorious traffic hot spots in York.
The final plus is after six months of using it and leaving the car at home I no longer need to fill the car up as often as I did and suspect it's paid for itself already.