I'd recommend joining the Onecnc user forum, tons of videos, posts and guys with years of experience.If you bought the software new, you have access to the forum and free support (Peter at UK support is great!). If not, I think you can join by paying a license transfer fee, best to check with support..
However, here are some quick pointers:
Importing: should work fine with stp. If the model's surface to be machined isn't flat in the XY-plane, rotate as needed or use the in car position buttons after setting up a plane. Machining boundaries need to be level, easiest way is to project them to any convenient z height (in a new layer maybe)
I've never had any problems with dimensions changing and I'd guess it's something else in this case.
Filleting: yes, it can be tricky. Make sure your surfaces are properly trimmed. There are some good posts in the support forum
Crashing: OneCNC is a little picky when it comes to hardware. Make sure you have the latest display drivers, this usually fixes crashing.
Welcome to the Onecnc gang, I'll be trolling around with others, happy to help whenever possible. To be of any real use, anyone helping you would need a file to look at/fix. Again, easiest through the forum, but you can also email me if you need help with a file.
(XR4 Mill Expert)