Hi,
You want to improve your health and good health is priceless, if you're going to get into cycling, and you should because it's the most enjuoyable way to get fit (in my humble opinion) then be prepared to spend a little money. Look for a decent road bike that you are comfortable with in your price range. If you see a better bike that's a little pricier then I'd buy it.
Look at it this way once you have forked out the initial expense for the bike and gear there's nothing else to pay apart from maintenance. Cycling gear doesn't have to be expensive and you don't need a super racing bike just a good road bike.
Cycling's like anything else, you can go mad and buy the best of everything and spend thousands or you can shop around and make buying decisions on your own needs not because something is supposed to be the best.
I look at plenty of bikes that cost £2000.00, I love looking at them but I won't buy one because I don't need one.
You're making the right choice to improve your fitness by taking up cyclng. you won't regret it. Good Luck
Decca