clarkbre

In all honesty, it sounds like you need to blueprint your future bike. Answer some critical questions and then start planning around the answers. You've told us you want to do some rail/trail and street riding and that you have an older cyclocross frame to build around. Perfect.

What wheel/tires size to run?
Run a set of 700c (622mm) wheels in the 15mm to 20mm width. For general use, 32 or 36 spoke wheels will be fine.
A good all-round tire size for rails/trails & road will be 700cx32mm. It's wide enough for comfort and light rough terrain yet narrow enough to run at a high pressure with low resistance on the street.

What cassette, crank, derailleurs & how many speeds?
Pick this question apart.

1. Analyze what terrain you will be riding on. Is it all relatively flat, steep hills, or rolling hills? Generally the flatter an area the less gear range you will need. I find a range of 30-90 gear inches on my hybrid is more than adequate and I ride a lot of hills.

2. How many gears do you need? Is more better or is simplicity bliss? You could go as simple as a 1x7 set up or as complicated as a 3x10.

3. How close do you want the gear steps to be? Slightly different or huge jumps? Close gear ratios are nice to maintain a specific cadence but the large steps are ok if you are keeping it simple and just out to enjoy the ride.

Once these are answered, you can start researching what parts will work with what. To start, focus on the number of speeds and what cassette/crank combination it will take to stay within the gear inch range. After that, figure out the proper front and rear derailleurs and shifters.

Sorry this is vague but these are the steps I've taken in building a few bikes. The outcome has been good and only needed minor adjustments.