I'm not too far from you and there are lots of places to ride in OC. The best spots I found are in and around the Irvine area if you ride the streets. There are also a good amount of dirt trails if you go north toward the canyons and Irvine lake area.
As far as a bike is concerned I have a road bike, commuter, and hybrid, but to be honest I ride the road bike 90% of the time. I sold the mtn bike because it was just too heavy and I really didn't plan on doing a lot of jumps and serious mountain riding.
I started off with the commuter because it was cheap, but found the road bike to be more comfortable so I never ride the commuter anymore. I would get either a dedicated road bike or a hybrid and just commute on that for the time being especially considering your budget is $300. I would lean more toward the road bike because it's a lot easier to commute on that and if you play your cards right you can get one that would allow you to put on some 28 tires which are more than enough to ride the local dirt trails on. I know it would be a hassle changing out tires, but this would give you the best of both worlds for very little money.
If and when you do make it out PM me if you want to see what it feels like riding a hybrid for a few hours or the road bike. This way you can try before you by so to speak.