Sounds like you are describing Revit, apart from the cross platform thing. - can link in/import a number of existing file formats - can cut sections as you des

Sounds like you are describing Revit, apart from the cross platform thing. - can link in/import a number of existing file formats - can cut sections as you described - API is well documented and in .NET which lets you program in C#, F#, VB, C++ and any other language which runs on the CLR. Great product, which improves dramatically in each release (so if you only looked at it a few versions ago, try again). It will cost you more than $1000 though!