Another newbie with some questions

Hello everyone, I'm trying to get Into cycling as another way to stay/ get in better shape and had a couple questions. As used newer road bikes are hard to come by in my local classifieds my options are somewhat narrow. I'll probably work my way up to at least 8-10 miles on routes with some decent hills. I want to spend as little money as possible but don't want to buy a junker. I was looking around and after a couple weeks of research, this is what I've come up with for options. Buy a new road bike (Cannondale Synapse maybe?)- $900 Buy a new single speed road bike (from State Bicycle Co.)- $430 (would this be a better workout or just too crazy?) Find an older road bike on CL and upgrade it- $? What do you guys think is the best option, I know a new multi speed road bike is ideal but even the $900 Cannondale (cheapest I could find new) is pushing my budget so if I could get a cheaper option that'd be great.Thanks!