You'll get all sorts of bike buying advice, but sometimes you just need to go for it. 
I slowly got heavier and heavier until my late 40s and decided to do something about it. I lost weight slowly, began cycling on my front suspension 
Halfords mountain bike, joined cycling forums and decided I wanted a road bike. 
More specifically I wanted a 2010 allez elite in red. A fairly silly buy, with an aggressive race oriented geometry, for an overweight older guy. It was £900 new, but I found a 6 months old one, which was still the current model on 
eBay for £540. 
I bought it, built my miles up slowly, but ended up doing about 12000 miles on it before it fell apart. 
I never once regretted it, in fact my more expensive bikes nowadays simply don't bring the same sense of satisfaction that one did.