Looking at your bike, I'd say that as you begin to "morph" it into a quasi-cyclocross bike, the first thing you should do is put on a pair of cyclocross tires, if, of course, you feel the wheels are strong enough. If not, see about some stronger wheels. You don't need to sink $3000 into a pair of Zipp CX wheels, but even something like $200 into a pair of Mavic Open Pros on any decent set of hubs will work.
As for the gearing, stick with what you have right now. The more you ride CX with your bike, the more you will have an idea about higher and/or lower gearing on your chainrings and cassettes. Some guys race single chainrings, many use double, and plenty use the stock setups thier bikes came with, like 48-39 up front and 12-26 on the rear.
Basically, concentrate on your wheels first, and the more you ride CX, the more you'll know what to modify or replace. This is how the modern CX evolved from the road bike, and unless you lay out the cash for a new CX bike, this is a good way to go. This is also how my old beater/training/commuting/playing-around bike became my first "CX" bike back in HS when I could only afford my road and mountain bikes to race on, plus one beater bike. My new, very real CX bike is a long-overdue gift to myself 12 years later.
I ride my bike on the road until I see dirt. My rides take me through short and long dirt paths that are everything from level to steep, across fields that range from grassy to muddy, through sand and just about every kind of surface the roads and not-exactly-roads that Washington DC can throw at a bicycle. As for the rough stuff, it's all about discretion. I'll bang and bounce over rocks and roots, but with finesse and skill rather than bombing them MTB-style.
I ride with a small Camelback, mainly because I don't want waterbottles on the bike if I need to dismount and shoulder the bike, plus it keeps other stuff like pumps off the bike and allows me to carry everything I need for a long ride.