I'd recommend either an endurance road bike, or if you really do a lot of gravel riding, a cyclocross bike.

Endurance road bikes are usually comfortable, not too aggressive posture, and often can fit 28c tires, which will do fine for light trails and gravel. They start around $600-800 new. Examples of these are the Giant Contend (previously Defy), or the Cannondale Synapse.

Cyclocross bikes are basically road bikes with much more clearance for tires and mud. I love mine, as it basically goes anywhere, but is still quick on road. They are generally a little more expensive, but not always. Common examples are the Kona Jake, Giant TCX, or CAADX.

Craigslist is a good idea to save money, as there are often 3-5 year old bikes that retailed for $1500+ going for $300-500. Cyclocross bikes are a little harder to find used as they have really became popular in recent years.

One thing I'm gonna recommend, and this is just my own opinion, is don't get a hybrid bike. Those are jack of all tirades master of none. A cyclocross bike will go anywhere a hybrid will, but much faster!

Hope that helps.