CrossRoads can be a huge help 
I agree, the electronics and firmware is the easy part. The enclosure is the hard part. Best to use an off the shelf enclosure if you can. If not, have a look at shapeways.com.
The hardest, and possibly the most expensive part, is certification. Europe and Japan need "CE Certification." Depending upon what you're building, you may be able to "self certify" but you may have to pay a "noted agency" to certify it for you.
In the US, if your clock speed exceeds some amount (I think 7 MHz) you'll need FCC certification. Similar deal.