I have already worked on projects like this one and I am listed in ATMEL's website as an expert in Atmel microcontrollers like Arduino. I have also been designing Arduino projects for many years. My company's main roll is designing prototypes and even do small runs (5 - 50) units. All our prototypes include custom enclosures so if you decide to go to production we can help you.
The database will be done in SQL server, if you require it, I can provide you with a free of charge license to SQL 2014 express for your project. The web end of the project will be made using asp.net and requires IIS installed on the receiving computer.
I have done multiple projects like this one for phone card companies and laboratory research.
All I need from you is voltage ranges to be measured, required precision and number of inputs. Also, do you have any preference on the microcontroller? I can use ATMEGA328 or ATMEGA32U4 unless you need a large number of inputs per device, the second one will allow us to create a USB interface in case we want to configure the units. Also configuration can be done via Ethernet (embedded web server).
The design will include a regular wired Ethernet connection, if you require Wi-Fi please let me know.
Alfredo