It shouldn't be that costly. My ex-wife used to sew one in all my jackets. Now I can usually get my sister to do it when I buy a coat, but if you have to pay it shouldn't cost more than about $25.
If you can't find someone local to sew for you, try Seams Like Magic at http://www.seamslikemagic.net/
As far as which design, use whatever works for you. I've used Ammars original pattern for about 20 years. I've seen Cloutiers and Berry's- they all work and I think it's mostly a matter of personal preference (although Jay Scott's is the only one I've seen that you can use in waist-length jackets.
> 2) Are there additional customizations to the basic Topit concept I should be considering? (For example, the ability to retrieve stuff from the outer side jacket).
Depends. Do you plan to do any routines where you need to retrieve stuff from the outer jacket pocket?