My husband and I bought bikes last year, and I thought we were going to need specially made bikes. We went to a bike shop and the salesstaff were so helpful. They pointed out the features we would need, which were mainly no fancy "spring" suspensions, instead solid posts wherever possible, and handlebars at a comfortable height - basically a sturdy old-fashion bike. The bike shop bikes were a little expensive, so we decided to wait to make that kind of investment after we learned whether we would enjoy biking or not. So, we went to Walmart, and found that they had several bikes that fit our criteria from $65 - $150 We bought the third cheapest ($79) and took them home. Hubby was comfortable with his seat, but mine was so horrible it actually caused physical damage (ok damage is too strong a word, but I was very sore in embarassing places for a week, Yow). I bought the widest gel seat that Walmart had (which wasn't all that wide, so I thought I might have to go on the internet to buy one even bigger), but it worked fine. I figured if I got one year of use out of it, I'd be happy, since we were kind of using them as just testers. But the bikes are holding up fine.
So, if I can ride a cheap discount store bike, with only minor adjustments, I don't think you need any kind of special bike.