I need a custom shopify app

General information for the business: We are looking to integrate the service offered on our server into shopify retailer websites
Description of requirements/functionality: Hello,

I am looking to get a custom shopify app which at heart does 3 things:

1. The app will automatically insert a JS script into every page of the shop
2. The app will detect when a visitor completes a purchase and it will a) make a JSON call to our server, and b) if possible call a JavaScript function defined in the JS script mentioned in #1
3. In making the aforementioned JSON call, parameters to the JSON call will be provided by the JS script mentioned in #1 - these parameters will probably be provided via hidden form fields added to the checkout form, although happy to consider any other means you may think is better

When a shop installs the app, there will need to be at least one configuration option that tells the app what the URL of the JS script (mentioned in #1) is' it will need to validate this to make sure it matches a specified pattern and that it exists. Ideally, the app would also make a JSON call to our server when it is first installed for extra configuration options, or better still the user could click on a special link on our server to install the app using configuration options set in the link (although I understand that either or both of these last 2 things mentioned as ideal may not be possible, you tell me!).

I appreciate this is slightly vague at the moment, I will give more details later. For now I want to know if you can do it for me?

In addition, I'm not yet familiar with all the details of shopify apps, for example where and how exactly are apps hosted, can they be hosted on our server, etc? Answers to these questions may dictate what is the best way to implement and install the app.

Martin

PS. In the future I would expect to port the app to other platforms, please let me know if you have experience of any of the other platforms

PPS. I have a couple of additional questions:
1) Can the app detect when someone views full details of an item?
2) Can the app detect when someone has added something to their shopping basket (ie. when their shopping basket is not empty)?
3) Am I right in thinking shopify may take you off-site for payment, and if so is that always or is it an option you select when you setup tyour store? By off-site I mean that products are added to your cart at www.acmeproducts.com but checkout is done at https://checkout.shopify.com