Which IFS Integration API's use / do not use Primavera API's

We used to use the IFS Integration module but stopped when we could not agree licensing for Integration or OPG with Oracle i.e. it was going to be too expensive for us.
We are currently looking at writing a custom interface (probably using APEX) but need to avoid the Oracle Primavera API's.
Rather than writing everything from scratch have you information on the IFS Integration API's :
What can we / cannot use by way of the API's, which ones call Primavera API's and which ones wholly use IFS code OR do you call some Primavera API's from within your code, if so what do we need to avoid?
I thought we might be able to call some of your code from say APEX to create a custom integration instead of starting from scratch.
Any information you can provide would be gratefully received.
I hope I have explained the issue sufficiently."