Thank you for your post!
Port forwarding is exactly what you'd want to look at. It will be necessary for remote users (outside of the network) to come in and be routed to the FileMaker server. For more information on Port Forwarding see Port forwarding information.
As long as your Accounting Software doesn't use Port 5003, I can't see that you'd have an issue. However, you may want to contact them to verify this.
I am also going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive even more views and potentially more advice!
The most important question you have to answer is.. will a technician ever need to use the solution from a location where there is no available internet connection or a seriously crappy connection. If that is the case then the only option is to deploy to iDevice and build in a sync routine.
You could also consider publishing your database such as via WebDirect such that your remote users can access the DB via a web browser.
This could be pretty simple to do or very complex to do depending on the design of your database and the needs of your remote users.