First - welcome to the Spec Focus brotherhood - have some of our cool-aid!
Love your plans - here are my thoughts on the Spec Focus build. The nice thing about the 2.3 motor is that you won't run into a lot of costs once the swap is complete - no internal motor work is allowed. It's plug and play. With the Zetec, you're really going to need the Ford Racing Head/Cams and intake manifold sooner rather than later. If you feel comfortable doing that work yourself, that may not be a bad way to go. However, I think you'll really like the 2.3 motor a lot better - more torque and less install headaches. Not sure what they run for around your area, but there are running examples in the Wash DC area here for around $700 or so. Cheap Cheap!
You have two choices in SCCA. You can run SSC (Showroom Stock), which requires a full interior, stock suspension, and a bunch of other stock pieces. Or, depending on the year of the car, you can run Improved Touring (ITS). As I understand it, the Focus is uncompetive in SSC - you'll be running against mini coopers, etc. and your build costs will be siginificantly higher to be even remotely competitive.
Your other choice is ITS. ITs cars have to be (I think) at least 5 years old, but a car built to Spec Focus specs can be reasonably competitive. Where you'd still be lagging would be in the suspension department, as ITS allows adjustable suspensions and higher spring rates than Spec, but you could expect to run mid-pack (depending on skill) after a year or two in ITS with a car that is 100% Spec legal. See www.improvedtouring.com
for rules and other ITS information. This would mean that your car might not be ITS legal until next year - but you could spend this year getting it built and running spec and D.E.'s this year and run both next year. FWIW, I'm in the same boat with my 2005.
ANother option might be to check out ITE - which in the DC area is the "Catch all" class that allows just about any car. Most of these are big 400hp turbo cars, but you're on the track and qualify/run with the whole field, so you're still racing with the same cars you would be if you ran with ITS or SSC. Check with your local region on what the requirements are.
If you have other questions, feel free to ask or PM, and I'll be happy to talk to you.