Cyclocross - How'd this work?

I'm lookin to try cross this fall and according to the Mrs., space requirements and finances have too many bikes already. And need a fairly inexpensive option to start.
Here is what I'm thinking and could use advise. I have an old Trek 850 Chro/Mo MTB with a rigid fork and canti-brakes, 7 speed rear with LX der. that is collecting dust.
My plan is some 1.3-1.5" tires, change the bar and stem to an old "dirt drop" from the parts bin. Shifters currently are thumbies, I can find some 7 speed SORA STI's inexpensively and run the original rear gearing for now. Any feedback?
What should I run in the front? A single for simplicity? If so what size? Or, a double with SORA STI? Or a Barcon? Or just leave the original triple for now?
Thanks for any suggestions......

You can certainly leave it and run a triple if you want. But if you are looking to change things up a bit, a single ring in the front will get you around the course just fine. I run a single ring 42 in the front and an 8spd 12-26 in the rear. I figure that if I can't spin that up a hill, I should be running anyway.

I set mine up on the cheap. It had a double (39x50) crankset and I just put a 42 in place of the 39 and ground the teeth off of the 50. A third eye chain watcher on the inside and the ground down 50 tooth ring on the outside keep my chain on the 42.

Thanks, I like the idea of the simplicity of a single front, rear would be 12x28, looks like I can do it on the cheap.
How bout crank length?