If you only want to ride a bike every so often around the neighborhood, then by all means go out and buy any bike you like. My grandparents recently found an old three speed at a garage sale for me that cost $5. I love it ... for riding around the block.
But if you're going to be serious about your riding, or think there's even the slightest possibility you'll want to get serious. I have one piece of advice for you: Spend the money to buy a bike that fits you.
When I got serious about bicycling I went to the local bike store and explained what I wanted and they helped me pick a bike. But I made a mistake. When I tried the bike on for size, the only measurement that I knew to check was the height of the top tube.
And from what I understand that will get a reasonable fit for folks with average body proportions. The problem is that I do not have average proportions.
I am relatively tall, measuring in right at 6-feet. But I have short legs. The only reason I buy 30-inch inseam pants is because most stores do not carry shorter lengths in my waist size.
So what I ended up with was a bike that was the right size for my legs, in other words I wouldn't bust my balls if I slipped off the saddle; but did not fit my upper body at all.
Riding long distances on the thing was not as comfortable as it should have been. I was constantly playing with the handlebars trying to make for better posture for my back, but I never could find that Goldilocks "just right." And the front wheel would often hit my foot while making a turn.
I'm not really certain what this means as far as my next "serious bike" is concerned. I've talked with some more folks and it seems to be the consensus that I need a frame with a longer top tube, to give my upper body more space to spread out in.
So when you're out looking for a bike, please go to a real bike shop and ask all the questions you can think of. And get a bike that really fits you. It'll really make all the difference in the world.