On bikes, if I were in your shoes I'd go for a good second hand bike as a first bike; and then I'd sell it (for close to what you bought it for - so a small overall cost) and upgrade if you want to in the future. If you do buy second hand, be aware there are lots of stolen bikes on the second hand market so try to get assurances that it's not nicked.
In terms of what type of bike - I bought a cyclocross bike about 15 years ago and its great for road cycling and also some track-based cycling. I've ridden it on B-rides in the club. So you might want to nose around these types of bikes. If you haven't already, head to a large Evans store and get a tour of the different options.
On building up fitness I would thoroughly recommend 'Heart Rate Training' by Benson and Connolly. I wished I'd read it years ago.
Best of luck.