No hills, no problem. Riding single will be a great simple way to get back into biking. I was on a 70s ten-speed with broken shifters when I started back up, so basically I was riding single until I got my bike converted to actual single speed and it's still my main ride. Find a bike that makes you happy and you'll love it. Gears are not necessary.

(This board is a little bit skewed towards fixed gear, so don't worry about those hooligans.)