I just began learning about CAD 3D modelling and I plan to invest in the future in adquiring a CNC router for hobby wood manufacturing of musical instruments. 

I'm currently learning Rhino (still on the trial period) and I'm leaning towards purchasing a license. I'm aware that I'd need MadCAM or RhinoCAM for the taking the necessary steps for CAM/CNC.

I've been flerting as well with Vectrix Aspire, but I have had zero experience with it. I know it has CAM capabilities built in, and that it may be a weaker modelling software, but a more straightforward CAM software. 

While I still sort that out, I have the following situation:

I am about to purchase a set of files (20 or so files) for a CNC routing project. The designer is offering me to send the files either in IGES/STEP, which would be the format he offers for Rhino and Fusion, or in STL/DXF format for Aspire. He doesn't seem keen in handing me both format sets. 

And I simply could not commit right now for a specific format because I could eventually end up using Aspire in my future CNC setup. 

Should I purchase the IGES/STEP set and use the Rhino that I have now to convert said files to  STL/DXF as a backup plan for the future? Would such a conversion, done in Rhino, render me quality files to be used in a future routing endeavour done in Aspire?

Another way to put the question, will the conversion be as simple as choosing some options within Rhino or would changing formats require some reenginering of the model for the correct application in CNC?

Many thanks!