QB Online API

Hi Experts,

I'm starting a new web dev project, still in the planning/proposal phase, and I need to integrate with 4 different applications: Google Maps, QuickBooks Online, Square and Zoho CRM.

I have integrated previously with Google Maps by using these samples, and found out that Google gives $200 non-transferable credit per month, which is for 28,000 loads per month, then $7 per each additional 1,000 loads.

For Square & Zoho, I'll post separate, but similar questions. This is particular to QuickBooks Online (QBO). The purpose of these questions will just be for me to determine difficulty, amount of work, and third-party costs, if any.

With QBO, specifically in customers, I need to:

1. Setup a custom field "type" - C or S.
2. From the API, retrieve customers based on their type, including name, transactions, and balances.
3. With the API, post transactions (bills/payments).

I did integrate with QB Desktop 15 years ago with VB6 & SDK, and I remember each customer has a unique ID, but I'm guessing everything else has changed :).

So my questions are :

1. Are the above all possible?
2. Are there any samples, documentation, how-to videos?
3. Is there a development test environment ?
4. Are there any extra costs for integration, or usage - like Google Maps has?

I'm planning on using here PHP, MySQL, JS and jQuery/JSON .

Thank you.