What you want to get from Xero? You have created a confusion, please do not award now.
I think you are aware that Xero API is support multiple operations. So, I first want to discuss with you to check what you want exactly. From my previous experience with Xero (I worked for invoice section, I mean as soon as an invoice raise in my developed site it creates an invoice in Xero. So, that first I created the contact in Xero) I can say you integrating Xero will not give you any pain. But have you aware about their limitation?
> There is a daily limit of 1000 API calls that a provider can make against a particular Xero organisation in a rolling 24 hour period for each organisation connected to. If you exceed this rate limit we will receive a HTTP 503 (Service Unavailable) response with the message.
> In addition to the daily limit, each OAuth access token can be used up to 60 times in any 60 second period.
> A single POST to the Accounting or Payroll APIs has a size limit of 3.5MB.
> We can create a maximum of 2 private applications against a Xero organisation regardless of the user that created the application.
So, when we will design the architecture we need to keep this thing in our mind so that we can do it properly. Now, I will wait for your schedule so that we can chat together. Please do not hesitate to ping it would not cost you a penny if you will discuss with me.
I would prefer to see your message in my inbox to start on the application.